As far as I know (and I admit that I'm no expert), there is no or at best almost no overlap between a DTM car and the production car it is supposed to represent. I doubt an "M4" DTM would use a production-based tub (if the rules even allow such a thing), or a six cylinder turbo (again, if the rules even allow such a thing).
BMW could (probably will) simply re-skin the existing racers to create an "M4 DTM."
Edit: I did some cursory research, and it looks like many aspects of the car (electronics, brakes, gearbox) are standardized across all entrants.
http://www.dtm.com/en/Technical-Regulations.html Currently, the series requires 4 liter V8 power, but that will apparently change in 2016, when the series will shift to 4 cylinder turbo power.
http://www.autosport.com/news/report.php/id/107154
So the point stands that the "M4 DTM" is basically a silhouette racer.