JDM
The GTS4 is essentially a poor man's GTR. When Nissan released the GTR, the Japanese folk who lived in the mountainy areas with the rough roads and snow and crap really wanted them, but couldn't afford them. Nissan built an RB20 powered, heavier version of the GTR. The GTR uses all the fancy aluminium parts for some panels, and running gear etc. The GTS4 uses steel. So basically: it's the standard GTST body (so not wide body like the GTR), with the ATESSA 4WD from the GTRs, single turbo RB20.
GTS4s are less common because it is hard to find ones that aren't rusted out due to all the snow and salt.