As Albert has said above, to qualify for Group 4 more than 400 cars had to be built which were fundamentally the same vehicle as used for rallying. FIAT actually built 406 131 Stradales, which were sold to the public.
The Stradale was a car that had more creature comforts than the Group 4, but only just
The road cars had seats in the back - albeit only two, and had a headlining and grab handles, etc. Also, the Group 4 had a roll cage and usually a few extra lights on the front
The early rally cars were very much like the Stradale, but over the years the competition cars were progressively modified in order to keep them at the top.
Can you see the difference? Usually, yes, but not always from the outside. Comp cars had a totally different dash/instrument arrangement too.
There are a lot of differences, too many to list here
Check out
www.abarth-gmr.be and go to the forum, where a full list of mods and spec has been posted up.
Albert, I thought the Abarth Stradale was only available in three colours - red, yellow and blue. What's the fourth?