It seems most data centers for colo charge based on a 1U server. I then see prices jump to 4U.

What about 2U or 3U servers? Just curious as I currently use 2U locally as they are easier to work with and run cooler.

Well, there's the...
1U = 1U
2U = 1U x 2
3U = 1U x 3
-method which I've seen before...

Chicken is right, you probably see the jump to a 4U price which probably has some sort of discount built in.

It's because 4U will hold a mini tower, so it's a popular option. Most servers that go into a collo situation are either 1U or mini towers, hence the jump from 1U to 4U.

Actually most mini/mid towers are 5U in size. :)