diff --git a/src/ja/twi4/AndroidManifest.xml b/src/ja/twi4/AndroidManifest.xml
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+++ b/src/ja/twi4/AndroidManifest.xml
@@ -0,0 +1,2 @@
+<?xml version="1.0" encoding="utf-8"?>
+<manifest package="eu.kanade.tachiyomi.extension" />
diff --git a/src/ja/twi4/build.gradle b/src/ja/twi4/build.gradle
new file mode 100644
index 000000000..912782397
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+++ b/src/ja/twi4/build.gradle
@@ -0,0 +1,12 @@
+apply plugin: 'com.android.application'
+apply plugin: 'kotlin-android'
+apply plugin: 'kotlinx-serialization'
+
+ext {
+    extName = 'Twi4'
+    pkgNameSuffix = 'ja.twi4'
+    extClass = '.Twi4'
+    extVersionCode = 1
+}
+
+apply from: "$rootDir/common.gradle"
diff --git a/src/ja/twi4/res/mipmap-hdpi/ic_launcher.png b/src/ja/twi4/res/mipmap-hdpi/ic_launcher.png
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diff --git a/src/ja/twi4/res/web_hi_res_512.png b/src/ja/twi4/res/web_hi_res_512.png
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diff --git a/src/ja/twi4/src/eu/kanade/tachiyomi/extension/ja/twi4/Twi4.kt b/src/ja/twi4/src/eu/kanade/tachiyomi/extension/ja/twi4/Twi4.kt
new file mode 100644
index 000000000..3dcf1637c
--- /dev/null
+++ b/src/ja/twi4/src/eu/kanade/tachiyomi/extension/ja/twi4/Twi4.kt
@@ -0,0 +1,240 @@
+package eu.kanade.tachiyomi.extension.ja.twi4
+
+import android.app.Application
+import eu.kanade.tachiyomi.network.GET
+import eu.kanade.tachiyomi.network.asObservableSuccess
+import eu.kanade.tachiyomi.source.model.FilterList
+import eu.kanade.tachiyomi.source.model.MangasPage
+import eu.kanade.tachiyomi.source.model.Page
+import eu.kanade.tachiyomi.source.model.SChapter
+import eu.kanade.tachiyomi.source.model.SManga
+import eu.kanade.tachiyomi.source.online.HttpSource
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.jsonArray
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
+import okhttp3.Headers
+import okhttp3.Request
+import okhttp3.Response
+import org.jsoup.Jsoup
+import rx.Observable
+import uy.kohesive.injekt.injectLazy
+
+class Twi4 : HttpSource() {
+    // The domain sai-zen-sen.jp directs to their main site rather than Twi4. It has to be /comics/twi4
+    override val baseUrl: String = "https://sai-zen-sen.jp/comics/twi4/"
+    override val lang: String = "ja"
+    override val name: String = "Twi4"
+    override val supportsLatest: Boolean = false
+    private val validPageTest: Regex = Regex("/comics/twi4/[a-z]+/works/\\d{4}\\.[0-9a-f]{32}\\.jpg")
+    private val application: Application by injectLazy()
+
+    private fun getUrlDomain(): String = baseUrl.substring(0, 22)
+
+    private fun getChromeHeaders(): Headers = headersBuilder().add(
+        "User-Agent",
+        "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/95.0.4638.54 Safari/537.36"
+    ).build()
+
+    // Popular manga == All manga in the site
+    override fun fetchPopularManga(page: Int): Observable<MangasPage> {
+        return client.newCall(popularMangaRequest(page))
+            .asObservableSuccess()
+            .map { response ->
+                parsePopularMangaRequest(response, page < 2)
+            }
+    }
+
+    private fun parsePopularMangaRequest(response: Response, hasNextPage: Boolean): MangasPage {
+        val doc = Jsoup.parse(response.body?.string())
+        val ret = mutableListOf<SManga>()
+        // One of the manga is a link to Twi4's zadankai, which is a platform for anyone to post oneshot 4-koma with judges to comment
+        // It has a completely different page layout and it is pretty much its own "manga site".
+        // Therefore, for simplicity sake. This extension (or at least this source) will not include that as a "Manga"
+        val mangas = doc.select("section:not(.zadankai):not([id])")
+        for (manga in mangas) {
+            ret.add(
+                SManga.create().apply {
+                    thumbnail_url =
+                        getUrlDomain() + manga.select("header > div.figgroup > figure > a > img")
+                            .attr("src")
+                    setUrlWithoutDomain(
+                        getUrlDomain() + manga.select("header > div.hgroup > h3 > a").attr("href")
+                    )
+                    title = manga.select("header > div.hgroup > h3 > a > strong").text()
+                }
+            )
+        }
+        return MangasPage(ret, hasNextPage)
+    }
+
+    // We have to fetch all manga from two different pages
+    // One from the homepage (which contains all ongoing manga), one from the completed manga page
+    // The menu at the top relies on JS which JSoup doesn't load
+    override fun popularMangaRequest(page: Int): Request {
+        return if (page == 1) {
+            GET(baseUrl, getChromeHeaders())
+        } else {
+            GET(baseUrl + "completed.html", getChromeHeaders())
+        }
+    }
+
+    override fun mangaDetailsRequest(manga: SManga): Request =
+        GET(getUrlDomain() + manga.url, getChromeHeaders())
+
+    override fun mangaDetailsParse(response: Response): SManga {
+        val document = Jsoup.parse(response.body?.string())
+        return SManga.create().apply {
+            description =
+                document.select("#introduction > div > div > p").text()
+            // Determine who are the authors and artists
+            // 作者, 原作 -> Author (Also the artist) / Original author (Such as light novel adaptation)
+            // 漫画 -> Artist only
+            // 提供, etc, etc -> Sponsors, irrelevant stuff
+            val staffs = document.select("#introduction > div > section > header > div > h3")
+            for (staff in staffs) {
+                val role = staff.select("small")
+                if (role.isEmpty())
+                    continue
+                when (role.text().replace("：", "").trim()) {
+                    "作者" -> {
+                        author = staff.select("span").text()
+                        artist = staff.select("span").text()
+                    }
+                    // If 作者 and 原作 appear at the same time, 原作 will overwrite the author field
+                    "原作" -> {
+                        author = staff.select("span").text()
+                    }
+                    "漫画" -> {
+                        artist = staff.select("span").text()
+                    }
+                }
+            }
+            status = SManga.UNKNOWN
+        }
+    }
+
+    override fun chapterListRequest(manga: SManga): Request =
+        GET(getUrlDomain() + manga.url, getChromeHeaders())
+
+    // They have a <noscript> layout! This is surprising
+    // Though their manga pages fails to load as it relies on JS
+    override fun chapterListParse(response: Response): List<SChapter> {
+        val doc = Jsoup.parse(response.body?.string())
+        val chapterRegex = Regex(".+『(.+)』 #(\\d+)")
+        val allChapters = doc.select("#backnumbers > div > ul > li")
+        val ret = mutableListOf<SChapter>()
+        // Returns the shortened series name from the request URL. Should be good enough for an identifier
+        val series = response.request.url.toUrl().toString().dropLast(1).substringAfterLast("/")
+        val sharedPref = application.getSharedPreferences("source_${id}_time_found:$series", 0)
+        val editor = sharedPref.edit()
+        for (chapter in allChapters) {
+            val match = chapterRegex.matchEntire(chapter.select("a").text())
+            val chapNumber = match?.groups?.get(2)?.value
+            ret.add(
+                SChapter.create().apply {
+                    if (chapNumber != null) {
+                        this.chapter_number = chapNumber.toFloat()
+                        this.name = chapNumber.toInt().toString() + " - " + match.groups[1]?.value
+                    }
+                    setUrlWithoutDomain(chapter.select("a").attr("href"))
+                    // We can't determine the upload date from the website
+                    // Store date_upload when a chapter is found for the first time
+                    // *Borrowed from CatManga
+                    val dateFound = System.currentTimeMillis()
+                    if (!sharedPref.contains(chapNumber)) {
+                        editor.putLong(chapNumber, dateFound)
+                    }
+                    this.date_upload = sharedPref.getLong(chapNumber, dateFound)
+                }
+            )
+        }
+        editor.apply()
+        ret.sortByDescending { chapter -> chapter.chapter_number }
+        return ret
+    }
+
+    override fun pageListRequest(chapter: SChapter): Request =
+        GET(getUrlDomain() + chapter.url, getChromeHeaders())
+
+    override fun pageListParse(response: Response): List<Page> {
+        val doc = Jsoup.parse(response.body?.string())
+        // The site interprets 1 page == 1 chapter
+        // There should only be 1 article in the document
+        val page = doc.select("article.comic:first-child")
+        val ret = mutableListOf<Page>()
+        // The imageurl is supposed to look like this /comic/twi4/comicName/works/pageNumber.suffix.jpg
+        // The noscript page broke the image links in a few mangas and they don't come with the suffix
+        // In this case we need to request an index file and obtain the file suffix
+        val imageUrl: String
+        if (validPageTest.matches(page.select("div > div > p > img").attr("src")))
+            imageUrl = getUrlDomain() + page.select("div > div > p > img").attr("src")
+        else {
+            val requestUrl = response.request.url.toUrl().toString()
+            val chapterNum = requestUrl.substringAfterLast("/").take(4).toInt()
+            // The index file contains everything about each image. Usually we can find the file name directly from the document
+            // This is a failsafe
+            val indexResponse = client.newCall(
+                GET(
+                    requestUrl.substringBeforeLast("/") + "/index.js",
+                    getChromeHeaders()
+                )
+            ).execute()
+            if (!indexResponse.isSuccessful)
+                throw Exception("Failed to find pages!")
+            // We got a JS file that looks very much like a JSON object
+            // A few string manipulation and we can parse the whole thing as JSON!
+            val re = Regex("([A-z]+):")
+            var index = indexResponse.body?.string()?.substringAfter("=")?.dropLast(1)
+            index = index?.let { re.replace(it, "\"$1\":") }
+            indexResponse.close()
+            val indexElement = index?.let { Json.parseToJsonElement(it) }
+            var suffix: String? = null
+            if (indexElement != null) {
+                // Each entry in the Items array corresponds to 1 chapter/page
+                suffix = indexElement.jsonObject["Items"]?.jsonArray?.get(chapterNum - 1)?.jsonObject?.get("Suffix")?.jsonPrimitive?.content
+            }
+            imageUrl = getUrlDomain() + page.select("div > div > p > img").attr("src").dropLast(4) + suffix + ".jpg"
+        }
+        ret.add(
+            Page(
+                index = page.select("header > div > h3 > span.number").text().toInt(),
+                imageUrl = imageUrl
+            )
+        )
+        return ret
+    }
+
+    // There is no search functionality in the site
+    // It is possible to implement something rudimentary for search to function
+    override fun fetchSearchManga(
+        page: Int,
+        query: String,
+        filters: FilterList
+    ): Observable<MangasPage> = fetchPopularManga(page).map { mp ->
+        mp.copy(
+            mp.mangas.filter {
+                it.title.contains(query, true)
+            }
+        )
+    }
+
+    // All these functions are unused
+    override fun latestUpdatesParse(response: Response): MangasPage =
+        throw UnsupportedOperationException("Not used")
+
+    override fun latestUpdatesRequest(page: Int): Request =
+        throw UnsupportedOperationException("Not used")
+
+    override fun popularMangaParse(response: Response): MangasPage =
+        throw UnsupportedOperationException("Not used")
+
+    override fun imageUrlParse(response: Response): String =
+        throw UnsupportedOperationException("Not used")
+
+    override fun searchMangaParse(response: Response): MangasPage =
+        throw UnsupportedOperationException("Not used")
+
+    override fun searchMangaRequest(page: Int, query: String, filters: FilterList): Request =
+        throw UnsupportedOperationException("Not used")
+}