Look to Europe, Australia and Asia;

Modern muscle included BMW M cars (can be 4drs), Saab Aero models could have been 4drs and hatchbacks, Mercedes puts a ton of HP into Sedans, Audi puts big engines into 4drs,
Australia has hot rodded many 4dr cars in their country, and Mitsubishi Evos are 4 drs as well.

The 2dr only muscle car limitation has been gone/going away for a long time. When I was in high school (and certainly, when my Dad was a young man) all the hot cars were 2 doors.
No one had a "cool" 4 door in my high school/college days. Lots of 2 dr Grand Ams, Dodge Daytonas, Plymouth Sundance Dusters, Chrysler LeBarons, Buick Regals, Olds Cutlass Supreme, Pontiac Grand Prix, Chevy Monte Carlo as well the usual Ford Mustangs, Camaros and Firebirds.
Slowly, things started changing. I remember the late 90s when you could get a Grand Am GT with a spoiler and in dash tach in a 4 door as well as the 2dr, or when the Supercharged V6 was available in the 4 door Grand Prix.
I also remember when Chrysler had 2 and 4 door midsize cars and toward the end of that run, the 4 doors were outselling the 2 doors at a rate of 4 to 1.
I remember the Pontiac GTO came out around the same time as the 4dr Dodge Charger (2005-2006). The Charger sold very well, the GTO not so much. The GTO was replaced by the G8 which was a 4dr. Too little, too late, though, as Pontiac was on it's way out.

Muscle is great, and I have a Challenger and I love it. But, I have 6 cars and room to store them all and the ability to maintain them all. I can see where someone with less storage and resources would prefer a Charger with 4 doors. Much easier to use the back seat and more room inside.

JS