More of the same
If you are a long-time fan of the Soul Calibur series, then IV will hold few surprises, but at this point, I don't think anybody is expecting a top-to-bottom overhaul. As you would expect, the main improvements are in the graphics. Characters are ludicrously detailed, decked out in insane, eye-popping outfits, accessorized to the max. The color palette is bold and vibrant. The camera can focus on closer objects now, making the backgrounds realistically blurry. If you have a hi-def setup, this is definitely a showcase title.
Gameplay is incrementally different. The biggest addition is the Soul Crush Critical Finish, although it comes into play so infrequently you might not even know it was in there if the game didn't go to pains to point it out.
The usual modes are present and accounted for. The major new mode for single player is Tower Of Souls which tasks you with ascending and descending a tower filled with enemies who have to be defeated by your crew of 1-3 (depending on the stage) fighters. It's got enough challenge and unlockables to keep you playing for a long time.
The major new mode for multiplayer is, of course, online. Although the interface could be generously described as "adequate", it is still a lot of fun to take on real humans. Your network connection will determine how much satisfaction you take from it though, if the ping between you and your opponent is too great, the game gets terribly laggy (press a button and watch your character respond much much later). Playing with a friend in the same room is still going to be your best bet.
The character creation mode is crazy deep. You can go strictly for looks, or get deep into an RPG-ish system for boosting various attack/defense attributes. Be warned that if you do go for the stats, you can end up with some bizarre-looking characters!
If you like fighting games, SCIV is a no-brainer. If you aren't big on the genre, you might want to rent it first.