In UC the four-mirror armor torso piece (piecemeal armor variant has armor split into arm, leg and torso pieces to form a total armor suit) is +5 ac. If you combine with chainmail you get a total of +6 ac, 5 from the four mirror torso, and +1 from chainmail arm pieces. Chainmail leg pieces grant no extra ac, and there are no four-mirror armor leg pieces. Now, if you are wearing a leg, torso and arm armor pieces you get an extra +1 ac for having a complete suit (doesn't matter what that they are from different styles of armor).
A non-piecemeal suit of four-mirror is +6 ac, whereas a piecemeal armor with four-mirror torso and chainmail leg and arm pieces would be +7ac...I think.
edit: The piecemeal armor rules are also up on the pfsrd.