Spending less than 400 is probably not possible.
http://http://www.bayoushooter.com/forums/showthread.php?53970-xdm-3.8-bitone-40-cal.
The subcomp on an XD is 3 inches... the 3.8 is in the XDm only... although this is a full frame handgun. But, here's a 3.8" Not sure of the OP's status with the listing but you can try it.
Actually they make an XDM 3.8 Compact now. Confused me before as well.
That said, you flat out ain't gonna find one under $400. A regular ol' XD40 subcompact is much more realistic in that price range.