I think this was true at one point but not for the past 5-10 years. Based off of using the site I feel like now a lot of things start on other sites (particularly smaller accounts on twitter), get aggregated and popularized on 4chan, and then get picked up on other sites (often regurgitated back to twitter). Knowyourmeme shows this for a lot of things that people typically attribute as original to 4chan. There was definitely a time when a ton of stuff originated on 4chan but these days everything is so interconnected with the same people posting on twitter, reddit, and 4chan that I think 4chan gets a lot of unearned credit