Fans of Persona 3, rejoice. An upgraded sequel!
Persona 4's an interesting game. It does almost everything better than its predecessor, Persona 3. Let's break things down:
The battle system undergoes some changes. Physical attacks now only have one category, there's a few new status ailments, your main character is limited to swords only, and you get control of every character. If your wish. I'm rather disappointed at the simplification of the battle system; different attack types kept things interesting in P3. However, most of the changes are welcome (I tend to keep my characters on command trends, still), and it's generally a better experience than P3.
Whereas P3 had a vague story (defeat Full Moon Shadows) that was only explained in the very end with little incentive otherwise, P4 has a more drawing story in the form of a serial murder plot. But in terms of character development, this game really, REALLY shines. P4 fleshes out the idea of a persona in a much better fashion. Without spoiling anything, a person has to accept himself to obtain a persona. Once again, social links present real problems happening to real characters (again, some exceptions involved). Overall, P4 shows a lot of polish. Atlus really learned from the development of P3.
Now, so far things have been good. In fact, I really only have one complaint about the game. The difficulty received a major tone-down. With the exception of the first boss, most of the game is a joke. I'm no masochist when it comes to difficulty, but I rather enjoyed the challenge in P3. P4's difficulty level is just set too low.
All in all, P4 is a great game. Almost every aspect has been refined from P3. However, the feel of P3 and the challenge it presented makes me put it over P4 by the smallest amount. Having said that, it's really not significant at all. In fact, both games are equally great. If you love RPGs, there's no reason to pass up on Persona 4.