Actually the series mostly changed at 3, when the protagonist became the "fool" and everyone else came with unique personas. 

It's true that P4 was by far the most lighthearted, but it had its own darkness, and even compared to everything before that 5 is really dark. 

[Hider=spoilers]
I mean, you start the game being captured, drugged and forced to sign a confession that will probably get you the death sentance, and the motive force of the whole game was you getting a criminal record for stopping a rape. Compared to 5, 3's "end of the world" plot line is pleasant.[/hider]