*Disclaimer: It is wise and necessary, that when learning Ger Torah [or any and all Torah for that matter], the scholar at hand should have a sufficient level of understanding of Biblical and Rabbinic Hebrew.
For those that continue to learn the sources [such as Ramchal] in a rudimentary level of English, [they] will continue to come up short in these sacred and holy words of wisdom from tradition and prophecy.
In short, if the Ger is simply only a convert, then all I ask is, "how do you like your Neveilah served to you?" [...which as we know, the Jew is to give the Neveilah as opposed to selling it as he does with the Nochri, i.e. x-tian still in the church.]
4th kahal...of Gerim. [Taken from Sifre - Midrashic Halacha composed by the Sages of The Talmud, and often parallels the Talmud itself. In other words, this is Chazal.
"B'Kahal Hashem" - [right paragraph that reads בקהל השם]...Rabbi Yehuda says 4 congregations exist; Priestly Congregation, Levite Congregation, Israelite Congregation, and Gerim congregation. [In this context, Gerim must be non-Jews while Israelite must contain converts, for the discussion involves Gerim not marrying Jews while converts as Israelites because now they can marry Jews] The Sages say, [the count is different entirely, leaving the entire system listed as a different concept] there are only three Congregations.
See the Torah Temimah who explains the positions of the Talmud and the outcomes of the Congregations/Houses of "Israel." It should be noted, that King David, explained by Rashi and especially taken up in later generations by such luminaries as Rebbe Nachman [see my shiur on that - Truly Ger] has many times in Psalms spoken of the Four Houses of Israel - Priestly, Levite, Israelite, and the Gerim.