Just like what Eiri is wondering:
So what do you think? Why does Riku stick with Force/Forth?

Maybe it's because Force is kinda different from the other nobility...? He doesn't really care about one's position in society [like how he's friends with people that aren't extremely "high class" and goes to parties that are beneficial to his position in society as seen in chapter 1]. Idk, that's what I think

i think that maybe Fourth is actually "playing" the role of the fool (in the family). i think that he only acts laid back and that in actuality hes planning something big ,like a take over or some sort of a secret mission,etc... and Riku is somehow connected to this ,or maybe he was taken by Fourth's charisma and wants to help him accomplish his goal too... maybe its just love at first sight ,fourth might have bribed him with food XD