Assassin's Creed and DMC4 combat systems having nothing in common.
DMC4 is a beat-em-up game where the gameplay revolves around combat and mindless action, Assassin's Creed is an adventure/stealth/platforming/action hybrid.
The combat system is pretty fun and cool with a sufficient display of brutal finishing moves. One of the best features of the game is the parkour-type platforming, especially when being chased by enemies looking for a hiding place.
Unfortunately, a large part of the game starts feeling repetitive by the time you've completed half of it (probably even before that).
Prior to every assassination, you need to do some investigations and these are pretty much the same EVERY time. You can do all of the investigations to get more information and be better prepared for the upcoming assassination, but you'll find yourself only wanting to complete the required three and leaving the rest by the time you get tired of doing the same stuff again and again. The good news is that the PC version has just a tad more variety with those than the console version.
The assassinations themselves are varied and are pretty nice, though.
Overall, it's a good game, but certainly undeserving of all the hype it got.