Same Engine RB20 DET, Same Chassis, Hicas is the same, the only differences are as follows
The GTS4 has got the GTR Altessa 4 wheel drive system, the GTS hasnt.
No turbo guage in the GTS4, you've got a Torque Split Meter Instead.
GTS4 is Quicker off the line due to the Traction, apparently!!
GTS4 is 60 ilo heavier due to the front drive train. SO slightly front heavy but not noticable really.
Mine has GTR Brakes on it, but some dont. they were an option me thinks.
Oh and the Badge!!!

But be aware that the R33 GTS 4 IS NOT the same as the R32 GTS4, Kempy has the R33 GTS4 and he can tell you about that....
Hope this Helps!
Si