Soulcalibur IV is actually the fifth installment in the series. It began as Soul Edge in the arcades and was renamed when it appeared on the original playstation as Soul Blade. The sequel had them renaming the series again to Soulcalibur when it was released in the arcades and it was later ported to the Dreamcast in time for it's launch. A remarkable improvement over it's predecessor, and sporting some of the best graphics at that time, Soulcalibur became an instant classic.
Soulcalibur II played it safe by refining the existing gameplay. It also added new characters, moves, and of course, enhanced the graphics. Soulcalibur III was a low point in the series and brought down the overall quality of the franchise. The game was riddled with problems and was practically unplayable for hardcore fighting fans. The game was loaded with glitches and characters weren't even remotely balanced. It didn't help that move sets were shortened and the character models were downgraded. Soulcalibur fans had already predicted this disaster because Soulcalibur III was the first Soulcalibur to release for home consoles without an arcade version to properly test and balance the combat. Soulcalibur fans feared the worst when they heard the news that Soulcalibur IV would again be released for home consoles without an arcade version to perfect it's gameplay.
I'm happy to report that SCIV is everything that it needed to be, and in my opinion, the best in the series. SCIV is a perfect blend of SCII and SCIII with a few next gen features to keep things fresh. Sidestepping in SCII was to fast and practically broken while SCIII's sidestepping was slow and almost useless. SCIV has found the perfect middle ground by offering effective sidestepping, but not so overpowering that you just sit and wait for an attack. Move sets are an extension of what was started with SCIII while the actual gameplay speed feels more like SCII, giving you enough time to respond to incoming attacks. The guard impact system has been improved to reduce those pesky guard impact battles that could go back and forth for quite a while. The guard impact window has been reduced and throws can no longer be impacted. You can also parry your opponent to the floor and back away if you choose to do so. A lot of work went into balancing these characters. With the exception of maybe Rock, everyone in this game is a serious contender and there doesn't seem to be a clearly overpowering character. Quite a feat for a fighter without an arcade release.
SCIV introduces some new features to the gameplay. The soul gauge, destructible armor, and critical finishers. Basically, the game rewards aggressive play and punishes overly defensive players. Constant blocking will deplete your soul gauge and cause a soul crush, leaving you vulnerable to the critical finisher. A move that can end the round regardless of how much life you have left. Also, if you take to many hits to a specific area, your armor will be destroyed and you'll take additional damage each time your hit there. The three levels of armor breakage are high, medium, and low.
If there's a better looking fighting game, then I haven't seen it. SCIV has the best character models I've ever seen in a fighter and the attention to detail on their main costumes is amazing. You could argue that the backgrounds aren't quite as impressive, but they are destructible and there's a really nice variety. The soundtrack isn't as memorable as previous games and some of the grunts sound a lot like hiccups, but the actual sound effects are fantastic. I also think that Tira's voice actor should win some kind of an award. She's very amusing.
SCIV won't be mastered as fast as previous entries in the series. The game is loaded with just frame inputs that require precise timing and attacks that have different properties depending on where you're standing or whether or not it was a counter hit. In other words, this is no button masher. With Tower of Lost Souls mode, the new skills system, character creation, and online play, SCIV has plenty to offer and I know I'll be playing it for years to come.