Why are classic 4 door sedans "disliked"?
For him, a muscle car means a classic American car with a V8 with dual exhaust.
Your nephew can obviously interpret things however he chooses and if he wants a 4 door, that's great. For a kid that age to show interest in older vehicles is the best thing for the continuation of the hobby so by all means go for it. However, for the sake of argument, I'm going to respectfully disagree with his idea of what a muscle car is.
Take it for what it's worth but if you look up up "muscle car" and click on the Wiki link the first sentence reads as follows:
"A muscle car is an American-made two-door sports coupe with a powerful engine, generally designed for high-performance driving."
That's a pretty big distinction. People have always bought 4 doors or wagons because they want cheap, practical and economical - no way they're spending extra on a racy look or performance gee gaws they don't need or want. The majority of cars manufactured are made for those reasons and the performance market is generally only a sliver of overall production. Even in a late model 'performance 4 door' configuration like the LX Charger, the amount of 6 cylinder or base engine models outsell the SRT/Hellcat variants by millions of units.
That's obviously why it's so hard to find resonable muscle-era projects these days - there just weren't enough of them made.
Enjoy your time with your nephew either way.