Soul Calibur 4 is similar to all the other soul calibur games. If you are happy with one of the others, I would suggest keeping that one instead of getting #4 (wait for Soul Calibur - Broken Destiny). There aren't really that many improvements aside from graphics, but it is nevertheless a great game. The best of the Soul Edge series, but not by far.
For those new to the Soul Edge series, it is a 2-player traditional fighting game with 35 unique characters and one buyable for a little spare change (5 of which have copied movesets). The fighting style of each character differs, but all are able to dodge, block, and attack. Blocks are either upper or lower body blocks, and attacks are upper, mid, or lower attacks. This makes the blocking system more interesting than that of some other fighting games like super smash brothers. There is an amazing "create a character" mode in which players may change almost anything about a character to make it his or her own. Online play has little lag and a very competitive community. It takes little time to get in a game, and spectator mode is also available.