I haven't gotten the Summon Night 4 OST yet, but I do have the Summon Night Sound Collection (which covers 1-3 in their entirety) and I can say it's one of my more favorite RPG soundtracks in recent memory. Much of it is extremely festive, upbeat and melodic, which pretty much describes composer Chiaki Fujita's style in all of the games (I'm unsure if she did SN4). It's not all like that, but that's where her specialty is and the focus of the music is(her worst and most generic tracks are when she attempts more dark and brooding music...she keeps these to a minimum, thankfully). Not too different from the Atelier soundtracks, though I think there is a higher number of instruments used throughout them (and *gasp* the tracks have actual endings!). The samples I've included are a pretty good indication of the rest of the soundtracks. She's definitely worth checking out.
Otaku.com sells the Collection for $40, which isn't bad, considering how they do shipping (if that's the only thing you order, you only pay $10 EMS shipping...anything else, you add $5 per item...which, frankly, still isn't bad, considering how bad shipping on 4-CD sets can be coming directly from Japan).