Why does flex not allow parties of 4?

There were instances where a party of 4 would bully the 1 and such situations. Stealing role report threatening etc

because 4 man parties proved to be dumb assholes to the one unlucky solo a lot more times than not. so you don't get to 4 man anymore because people lack basic etiuquette and ergo ya'll don't deserve to get to queue up as a 4 man if you can't treat the one extra wheel giving you the opportunity to complete a 5 man team correctly. due to this lack of etiquette flex queue started suffering from a lack of solo players to fill out those 4 man teams, so riot took measures and they've worked for the most part, aside from the general piss poor matchmaking present in flex.