That's interesting! any more information? And was it online only or local too?
I don't have that many controllers, so I could never test it locally, but I think you should give it a shot, haha!
There's like, a preferences.ini file, or something like that, in the folder where all the Risk of Rain data is. It's really easy to read/go through, and one of the lines says something like
playerlimit=4
and all you have to do is change the 4 to an 8, save the changes, and then you should be good to go.