Flinch covered all the differences but two. The "Tactical" can be hard to find and is a little more expensive. :wink:
I own a 4" XD9 and love it. I'm at about 800 rounds through it, Winchester White Box 115, Remington UMC 115, Blazer FMJ 115, Corbon 125 +P JHP and Federal 135 Hydra. Zero problems with the exception of one FTE due to an after market magazine for Beretta. With factory mags this gun has been 100%.
Also another difference, the 4" is easier to find holsters for. I use a Sidearmor (
http://www.sidearmor.net ) IWB and have a OWB modular on order. There are alot of other holster makers starting to make holsters for the XD also, Don Hume, High Noon, Tucker, Blade-Tech, etc... etc...