Most Group 4 cars of the era didn't have a spoiler and personally I think they look rather aggressively good for it. However, many have since acquired spoilers for aerodynamics and if you plan to drive over about 150 mph then a spoiler is recommended and most do opt for something similar to the GT5 style as it blends well with the flares.
Take a look at Patrick Hals Group 4 cars (Candy & Andy) detomasogr4.com
VDV Grant in Belgium has GP4 for sale too vdvgrant
This one has something more subtle www.boldride.com
Other than that it would probably be the small chin spoilers as fitted to may std Panteras.
Larry Stock at Pantera Parts Connection in Carson City has 3 or 4 spoiler styles on his wall. I looked through his CD catalog to see if he has photos, but none. Give him a call and get him to send photos of the various styles. He has molds for just about everything available.
Julian
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