Purely speculation on the controls. I just made this up:
Lmb=Left attack
Rmb=Right attack
Alt/Mmb=Block
If you had a shield in your left hand, then blocking would still be done with ALT, and the bashing with Lmb. I'm not sure why people think that you need 4 buttons. Why would you be attacking while blocking? Everyone would just tank behind their shield.
**edit
I looked it up andfound out that you click left to attack and right to block on pc they could have the left and right clickers use the weapon in there coorisponding hand and a key to press that changes them to block