4/20 may very well represent Mills' potential value in the open market, but I don't see much chance that he will get that sort of money from the Spurs. For the Spurs to offer that sort of money to Patty, you have to imagine a massive overhaul of the roster that includes trading Parker and making Mills the starting PG, IMO.
In the absence of major changes this summer, I believe that he'll have to be willing to accept something on the order of 3yrs/9M if he wants to remain with the Spurs.
Edit: There was also a recent thread that covered much of the same ground. You may find it interesting:
http://www.spurstalk.com/forums/showthread.php?t=230999