Open-source intelligence analysts reported that Crooks didn’t appear to use mainstream social media channels such as Facebook or Instagram and had an almost negligible internet footprint, which is highly unusual for a 20-year-old. Instead, analysts said that Crooks corresponded with friends and associates on a Discord server that they were now trying to access.
Discord servers are restricted ring-fenced sections of the internet. They are often used by gaming groups and communities as secure chat rooms but are also used by fringe organisations to push their ideologies and discuss wild conspiracy theories and plots.
I'm pretty sure I remember moot himself saying that he cooperated with law enforcement when they demanded IP adresses every time some 4channer broke the law.
It was from a court transcript somewhere, but you're absolutely correct in assuming 4chan is heavily monitored and participated in by law enforcers, government officials and far right recruiters.