I really hope P5 is darker in tone, it seems like it, but its too early to tell.
Persona 4 was fucking great, what are you talking about?
The idea behind Persona 3 and 4 is decent, but so far I found the execution of those games to be rather... sloppy. Biggest issue I had with P4 was the dungeon crawling, especially after just having played a certain other Atlus game that handled dungeon crawling and combat so much better (SMT: Nocturne)
P4 had very good gameplay for a story fag game: dem boss battles and you could actually get annihilated and loose 30 minutes worth of progress to a random battle against same level enemies (if you happen to fuck up and/or they have null/drain or worst of all, reflect immunities and nasty spells/spell combinations). You don't see the that kind of stuff in Bioware&Final Fantasy games or even more esteemed games like PST/Arcanum/Fallout.
Though if that's not good enough for you, there's always SMT X FE coming up.