SO4's gameplay is fine. It seems like a noticeable step down compared to SO3 at first, but that's mostly cause its different and unintuitive; once you get use to the game (which can take some time, but the first few areas of the game are appropriately weak...barring the first boss...so you have plenty of time to get yourself use to it, by the time it starts mattering), its enjoyable.
The game does pull a bullshit stunt with its gameplay though; there's this system called Rush. Rush is essentially the game's equivalent of Tales Overlmit; in Rush, your crit rate sky rockets, and you become immune to Flinching.
So what's bad about it? Enemies can do it too. Enemies will pull it off unexpectedly, asininedly, while you're in the middle of this big combo, and suddenly you get slammed for big damage. It doesn't hurt that much in the sense of "Loss vs. Win" but given the way the bonus gauge works, being hit ONCE by a Rushed enemy completely kills your gauge, and there's often no real defense against it; even if YOU dodge it, the AI may not.
As far as other gameplay factors go...
NOTE: I'm using Spoiler Tags of sorts to avoid giving away who joins your team, so don't take some of these descriptions TOO literally.
The characters are varied enough. Unlike SO3, you get 3 Mages, not just 2, so you have a few more options in that regard, and like SO3, Mages are functional as controlled PCs, for all that its advisable you don't control them for healing purposes (or alternatively, use 2 Mages, which isn't necessarily a bad idea.)
Your main is a typical Tales of Star Ocean Swordsmen; he doesn't have the token Ranged Slash move, but that wouldn't really work well in this system anyway, so its minor; good at comboing, well balanced, what have you. The typical character whose simple, but effective.
Token Female Best Friend Stereotype is an archer, so ranged oriented and good from attacking at a distance though naturally, has issues if something gets close.
The Cat-girl is your spammer, perse; she hits a lot of times, and combos, and keeps things tied up. She's also fast at running. Its what you'd expect of someone with her design!
Mega Man is about AoE nuking and just being a general powerhouse. He's slow, awkward to use, but the shear mayhem he'll cause to groups is absurd...thus good for AI! Though YMMV on the controllable factor.
The Grim Reaper is basically about running into frays, destroying everything, and getting out without being hurt. I know this sounds broken, but that's the best way to describe him; laser scythes have this effect <_< He also has limited attack magic! ...that you'll probably never use...but hey, ITS THERE!
There's a 9th character whose an attempt at being a Fighter Mage, but really fails at it, and should be ditched the instant you get a 5th character. Mostly cause he stops learning good magic, and gets no Physical skills...great combination there! granted, he's also a temp, replaced by another character, so its not a big deal
The Item Creation is more intuitive than SO3's. Its basically just "Have characters set up, they'll sit there thinking of idea,s and eventually something interesting will come up." To create items, its a typical Synth system; get x amount of y product, use them at Item Creation, you get item! It doesn't require any of this complex SO3 stuff, at least.
Oh, it also lets you take whatever item, and shove it onto pre-existing equips to slightly boost their stats. These boosts aren't major, though I suppose they stack if you do them enough. We're talking like "+4 Attack" when your weapons are in mid 3 digits.
