Of course 4chan is not just /pol/ but it is the biggest board, and together with /b/ contribute to plenty of hateful content as I mentioned. The culture between boards is different but /pol/ refugees in particular have been spreading to other boards for several years now and it's very annoying because even if a small group of them decide to visit a board regularly then they can ruin the culture because of relative sizes between the boards. Reddit and Twitter have their own problems, particularly with echo chambers but the biggest subreddit on reddit isn't spewing anywhere near the same kind of shit as the biggest board on 4chan does.

Go to any board, /lit/, /g/, /fa/ (maybe less so), will all have a thread or two that you will be able to tell are directly influenced by /pol/ posters. Some boards call them out, most don't.