I guess my view depends on its intended purpose and also cartridge size. For me there is a clear delineation between target/precision rifles (range guns) and field rifles. Weight is less of a concern for a range rifle that will be shot off a bench or bipod and only carried 50 feet from the car.
For field guns that will be carried then the distance carried, type of terrain, and shooting volume all become deciding factors... and also age/physical condition of shooter.
A 4lbs AR15 may seem too light, but it is a small cartridge and if outfitted with a muzzle break or suppressor then there is minimal recoil. The hardest part is getting an AR to reduce that much weight
