To do a critical finish you must first do a Soul Crush. Meaning your opponent will block, their meter goes from blue to green to red and then "breaks"...you have a split second or so to do the Critical Finish (all four buttons...or in the case of the PS3, L1...don't know what the X-Box is). I did one with Siegfried...it can be tough though considering a lot depends on your opponent.
Soul Calibur 4 is a beautiful game...and I was totally "wow'ed" by the graphics...both the cinematics and ESPECIALLY the in-game battles. I remember when Soul Calibur came out on the Dreamcast and I was amazed by the graphics then! SC2 and SC3 on the PS2, Gamecube, X-Box still had great graphics, but they had a more "anime" look to the characters faces and the graphics didn't blow me away.
As soon as I started playing, the graphics of SCIV instantly reminded me of that semi-realistic look that the Dreamcast had and I was excited to see everything move and act so fluidly...it is a beautiful game...graphically, design-wise, everything is gorgeous from the characters to the levels to the movement.
Though you can say that the Soul Calibur franchise has evolved little since its inception, it still continues to be one of the best with such a unique elaborate array of characters that only a few fighters can match (Street Fighter, Mortal Kombat, and Tekken to name a few).