So Ive maybe put in an hour or two of Innocent Sin...this isnt all that dark. Its about as goofy as P4 was at this point. Secret agent restaurant owner? Steven Segal is Lisa's dad? Statues walking around and everyone being nonchalant about it. Eikichi cant stay serious for a second, Lisa always trying push his buttons and get lovey dovey with Tatsuya. The fat girl being Michel's ex was pretty lol.
I mean the darkest thing thats happened is those three kids losing their dreams and ambitions but eh, its not murder.
Still, its interesting I guess.
The tone of Innocent Sin is definitely more serious than P4, but like all of the Persona games except for P1, it has many silly moments to balance it all out. I'd say the ending is particularly what makes the game darker than P3 and P4. Fundamentally, though, I feel that IS, P3 and P4 are very similar when it comes to the overall mood.
Where that significantly changes is Eternal Punishment.