My friend
Chooie has a 4" XD .40 and uses a
UBG Canute, he swears by it.
I have several UBG rigs and love them all.
You may also want to check out
HBE, Comp-Tac, and
Galco.
However, if you're going to go IWB with the XD .40, I'd suggest a IWB with the offset loops like the Milt Sparks VMII, UBG Canute, HBE Com 3, or C-TAC.
There's also a lot of good feed back on the Comp-Tac Minotaur and the Crossbreed Super Tuck, but I have no first hand experience with them.
Milt Sparks has a long wait period, HBE and UBG seem to be running around 2-4 weeks last I checked. Comp-Tac can get you gear in about 3-7 days on average.