The last 10 4th picks are:
2014 Aaron Gordon
2013 Cody Zeller
2012 Dion Waiters
2011 Tristan Thompson
2010 Wesley Johnson
2009 Tyreke Evans
2008 Russell Westbrook
2007 Mike Conley
2006 Tyrus Thomas
2005 Chris Paul
These results indicate a 20% chance at a superstar. From there, you have Conley, who is outstanding. Gordon has potential and Evans puts up decent numbers, but you could easily argue that the rest of this list isn't significantly better than either Bradley or Sullinger.
This is an excellent draft, and I believe a major star could be had at the fourth slot. But New York has less leverage than most of the posters here are claiming.
As others have noted, New York can't keep this pick. Its hard to see how they can afford to wait on a very talented 19 year old, like Winslow, Mudiay or Russell. They have real talent, but it is naive to assume they will be all-stars before they turn 23, and by then Melo will be lucky to still be a starter, much less a superstar.
So it will come down to how many teams go after this pick and what they are willing to bid for it. I doubt anyone will give them an all-star for it.