4 gag manga is a pretty normal number for this magazine, remember for a few years last decade we had Kochikame, Gintama, Saiki, and Isobee all running together. If you go back a little earlier, you had Sket Dance and Inumarudashii alongside KochiKame and Gintama, and before that you had Kochikame, Jaguar, Bobobo, and Mr Fullswing running together. Four gag manga at the same time is normal for WSJ.
Generally at any given time we have 2 pure gag (Kochikame & Jaguar/Inumarudashii/Saiki), one gag-action (Bobobo, Gintama), and one other mixed genre (Mr Fullswing is gag/sports, Sket Dance is gag/drama). Or maybe 3 pure gag and 1 gag-action.
Currently we've got 2 pure gag (Mitama, Moriking), one gag-action (Mashle), and other other mixed genre (Agravity Boys as gag/sc-fi adventure). All 4 sticking around wouldn't be too weird for the magazine. Though maybe Moriking will start fighting, who knows.
It only seems like a lot since the pure gag stuff never got translated much, so people ignored them in the ToC (and of course since we'd had periods of no gag manga recently after all the long running ones ended close together).
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