If I remember correctly, 一週間フレンズ is partially 4-koma. And also シメジシミュレーション is. Both of those were read in the beginner book club (though I didn’t read either of them).
In any case, ひとりぼっち is a great choice for a first 4-koma manga. It’s relatively easy (even considering the lack of furigana) and not very dense, but is still really funny and has great characters. I’m not sure how much the lack of kanji would hold back some people, but at level 51 on WaniKani that aspect shouldn’t be a problem for you.
Other than that, most of the 4-koma manga I read are from Manga Time Kirara. Specifically, my favorites are:
- ご注文はうさぎですか (all time favorite manga)
- こみっくがーるず
- ブレンド・S
I’ve tried a few more too. Some I dropped because I didn’t like them. Another only has one volume so far, so not really worth mentioning. Even if some don’t land for me, Manga Time Kirara is the only magazine (really three magazines) that I specifically go to for recommendations.