Below is a page from a 1976 Brochure
Another page from the same 1976 brochure. 4 Pantera versions marketed.
Unfortunately no 1977 or 1978 brochures were available to me.
Below is a page from a 1979 brochure; still 4 Pantera versions marketed. The Gruppo 4 has conspicuously been renamed a GT4. It would be natural to assume the GT4 and Gruppo 4 were the same car; after all they used the same picture in this brochure that had been used in earlier brochures. Except we know for a fact De Tomaso produced for a short time circa 1979 wide body street cars, based on the GTS, which they referred to as GT4s.
So instead of the "Gruppo 4" being renamed, what in reality transpired in the 1979 brochure was the GT4 was "added" to the line-up and the Gruppo 4 was dropped from the line-up. Without 1977 or 1978 brochures its impossible to say exactly when the Gruppo 4 was dropped.
In the small print the GT4 tires are described as Pirelli P7s, not racing tires. This car of course had 285mm front and 345mm rear street tires. The widest street tires ever manufactured. Porsche and Lamborghini were doing the same thing at the same time.
Below are two pages from a 1980 "GTS" brochure.
There's lots of detail information about the GTS.
The other Pantera models receive an "also mentioned".
There are still 4 versions of the Pantera being marketed. The GT5 makes its first appearance, and the GT4 is still there too. The Pantera L has been dropped from the line-up. Unfortunately I don't have a 1981 brochure available to me. I'd bet mention of the GT4 was dropped in 1981.
Below is a better view of the 1980 brochure, you can read the blurb about the tires a little better. Not a lot better, just a little better. 
By the way, can anyone explain to me what's the meaning of the word pointed out by the red arrows?
Finally below are two pages from a 1985 brochure, the next brochure available to me. By 1985 only 1 Pantera model is really advertised; the GT5-S. The GT5 and GTS are pictured, but there's no information provided about them, no specs. Its as though De Tomaso was showing pictures of the "out going" models. There's no mention, not even a picture, of a Group 3 model.
