I'd take del Potro over Murray any day. I think Nadal might actually be right if del Potro plays consistently and isn't hampered by his wrist injury.
Maybe Nadal would fall out of the top 4.
yeah the guy who is likely to dominate an entire section of the tour (clay) and is a strong favorite in most events will fall out of top 4....
the same guy who handled Murray throughout 2011 and Federer on non-indoor surfaces.
if Del Potro breaks top 4, either Murray or Federer will be 5, at least in terms of liklihood