I checked Major Nelson's blog, but no numbers as of yet. Perhaps the game's placement on the iTunes App Store and XBLA "top sellers" listing will give some sort of idea?
iphone version update:
Sonic 4 is #18 on the list of top paid apps in the "arcade" category. Also, Sonic 4 is the most expensive ($9.99) of all the games up to that point on the list.
Sonic 4 is #27 in the "Action" category. Again, Sonic 4 is the most expensive of all the games up to that point on the list.
Sonic 4 is #65 in the "All Games" category and Sonic 4 is the most expensive of all the games up to that point on the list. #1-64 are mostly 99 cents, with a few $2, $5 and $7 games.
So in all three "top selling" game categories that Sonic 4 falls under, Sonic 4 is the #1 selling premium priced game ($9.99 and up).