In my opinion the UD4 doesn't offer enough extras to justify the difference in price. The UD4 has a few extra ports; two USB 2.0 ports and an extra eSATA (which makes 2 eSATA that can be used for RAID 0 or 1, kind of cool).
The UD4 also has "GIGABYTE's cutting edge 16 phase power VRM design" as opposed to the UD3H's 7 phases. I am not exactly sure how important this is, but supposedly it can makes the motherboard more durable and longer lasting by putting less stress on each power phase.
On the downside, the UD4 requires a discrete graphics card, and therefore can not utilize the on-die graphics from the core ix-xxxx series (Intel HD graphics 2000/3000). I own the UD3H and I really like if something goes wrong with my video card that I won't be sitting around unable to use my computer while I am waiting for a replacement. I haven't got around to trying out VIRTU yet but I look forward to it.
Bottom line, I don't see enough advantages to go with the UD4. Obviously I am little biased, since when I was faced with picking out a board I went with the UD3H, but I figured I would put my two cents in.