What are the four most recognizable rule34 scat characters (or, personal faves)? I kinda want to draw (non-nsfw) scatbooru Demon Days parody art.

Not to off topic but pretty much any artist who draws a lot of scat with the same character falls into eh category. The most popular R34 characters/series could fall into it easily just from popularity alone.

Doujin content comes to mind, especialy if you measre by volume alone
Theres alot of Neon Genesis Evangelion stuff for example
nhentai.net/search/?q=neon+genesis+evangelion+scat

There's also a bunch of girls und panzer stuff, although its mostly limited to one or two pages per doujin, but this tends to be the case for the vast majority of scat doujin stuff in general ;-;

Outside of R34 stuff tho

Milele's Matsuyama comes to mind

Or Ameyama Denshin character in these doujins
nhentai.net/g/205242
nhentai.net/g/204070

Or Mikuni Atsuko's characters in any of her main series and so on..
nhentai.net/artist/mikuni-atsuko