blind-elk on the forum owns an RB25DET powered GTS4 and a couple of guys own RB26DETT powered R32 GTS4 sedans (coz they're cool GTR sedans)
R32 GTS4's are good fun in their own regard... I own one, light mods, still an RB20DET, plenty of fun in AWD mode and RWD mode (you can very easily wire in an override switch for the AWD)
GTS4's share a lot more common parts with a GTR than a GTSt, like the chassis for example, and the control arms etc. Swapping in GTR parts is pretty simple. I run aftermarket R32 GTR shocks and wheels on my GTS4, and even things like quick shift kits, control arms etc are interchangeable, which is awesome.
That said it s a bit of a pain in the freckle to sit there and bring a GTS4 up to GTR spec, when you can just spend about $5000 more and just buy a GTR in the first place. So maybe rather than trying to make it something it isn't, use it without modding it, when you have enough saved up, sell it and upgrade to a GTR. That's my plan anyways.