4 is one of those games that fist seems really kiddy and immature, but upon further consideration is much more well thought out than it initially seems. The Kids vs Adults was really annoying my first time through, but when considering the game outside of, er, the game, it's a very sound theme, especially for the Japanese audiences where growing into a more traditional adult means that your freedom and dreams tend to be shed for responsibility. The point really seemed to be showing that there are benefits to different lifestyles and decisions and sometimes one group can benefit from the other. There is no right answer to everything and you have to look out of your comfort zone to find it.
I'm not sure if it's a message that resonates as well with Western audiences.
4's ending was depressing as hell, as well. It's interesting, because despite 4 being very much thematically about being open-minded, it also is very fatalist. You cannot escape becoming an adult, just as you cannot escape death.
I actually didn't really like 5. At all. I'm not sure what I didn't like about it, in the end, either.