I looked it up out of curiosity. The post acknowledges, or at least implies that 4chan users use more slurs, and doesn't touch on racism otherwise.
For my part, I don't think there's a large gulf between racist opinions on the two platforms, just on decorum regarding slurs.

Here's the link: https://web.archive.org/web/20210922182734/https://www.reddit.com/r/unpopularopinion/comments/pt2gel/4chan_is_actually_more_welcoming_than_reddit/

[–] Traister101@lemmy.today 7 points 3 weeks ago

Ah, r/unpopularopinion one of the subs I almost never visited cause 90% of the stuff on there was just plain stupid.

For my part, I don't think there's a large gulf between racist opinions on the two platforms, just on decorum regarding slurs.

Yeah. I'd probably make the argument that 4chan is just open about the racism while other platforms like reddit have at least some enforcement leading to it being less blatant.