IF you were to speed up the courts and making conditions more polarized as they SHOULD BE.. You wouldn't have a top 3-4 anymore (since Nadal, Nole or Murray can't play as well on the fast stuff). Polarized conditions, there wouldn't be a consistent top 3-4 Period.
There would be a real top 10 in the hunt possibly. Since upsets would be more plentiful
It wouldn't be as stable, certainly, but with 4 great players it would probably still be pretty consistent. Murray & Fed would be great on faster surfaces and we all know how good Nadal & Djoker are on slower. Borg, Mac, Connors, Lendl were pretty stable while they lasted despite Mac & Connors's relative mediocrity on clay, Borg's struggling on outdoor hard, & Lendl's shittiness on grass.