It's alright. I first played it a few years ago when they ported it to Mac and Linux, uprezzed all the textures, and tweaked the last level, so comparing that, which almost is like a remake, to the PS4 version is like comparing a PS1 game to a PS2 game.
There's framerate drops from time to time, nothing incredibly terrible, it catches up in a few moments, but the one thing that's most noticeable is that the PS2 version (that this is a slightly uprezzed port of) has some major lighting issues. In game engine cutscenes have reflections on Raz's face that should make it brighter, but end up looking like incredibly bright shiny squares. That's the only problem that I find to be terribly egregious, as it looks bad, and clearly isn't the intent. It's not a fault with the PS4 version I believe, but rather of the original PS2 version that this is emulating.
It's also widescreen, on a version that wasn't intended to be widescreen, so the usual wider camera quirks pop up (characters just appearing in the middle of the frame, where previously that area of the frame was cropped out), and pre-rendered cutscenes are stretched to 16:9 rather than native 4:3 (this is true of the PC version as well)
As for the gameplay... it definitely hasn't aged super well, something I touched on in
this thread that got no love the other day. It's not terrible... just very quirky, and often times a little frustrating. The writing is the saving grace of the game, and if you haven't played Psychonauts, you're in for a treat.