The differences according to the Gigabyte site between the P67A-UD4 and the P67A-UD3:
The UD4 has:
-Support for Dolby Home Theatre
-Support for CrossfireX and Nvidia SLI (UD3 has only CrossfireX)
-Second Graphics slot runs on x8 (x4 on the UD3)
-Extra Marvell 88SE9128 chip (controls 2 E-sate 6gb/s, support for RAID 0 and 1)
-14 USB 2.0/1.1 connections (12 for the UD3)
-A second Renesas D720200 chip for up to 4 USB 3.0/2.0 connections (2 on the backpanel and 2 with connectors) vs only the 2 on the back on the UD3
-the UD4 also has 12 Phase power (couldn't find how many there are on the UD3)