Well even though I haven't tried it, isn't 4G supposed to offer longer string life though a reduction in tension loss? Someone like Federer doesn't have to worry about tension loss in a poly because he can just get a new racket delivered to him. Compared to ALU rough I believe 4G has less spin and is very low powered, so for Roger I'd assume the cons outweigh the pros.
Head Extreme Pro 2.0
Luxilon Big Banger Original