Nicky as always has been played a role as a sweet, innocent role who often doesn't get in trouble. However they had to ruin it in Season 3 when he made up the Meh-Guy. Also his reasons didn't make sense to why he told that his reviews need to be 'honest' when he hasn't been honest to them anyhow.
Ricky has lost his intelligence. If you remember about when they thought one of them wasn't a real quad. I mean who would think someone isn't related to them because of what their preferences are. I was shocked when he thought of the reasons why he didn't think Dicky wasn't a Quad. The other 3 reasons were more accurate than his.
Dicky was the closest and he is the dim-witted of the quad. Also I think he has lost his vain personality. He no longer talks about his good hair. Also, is less romantic.
Dawn has gone more goofier and jealous now.