I think this is a fair question because if you look at his numbers last year.
2011 Skelton's Numbers:
Look at this projected numbers if he played a full season.
Skelton's projected number if he played the entire 2011-12 season
These number's don't look nearly as impressive, but considering that Skelton posted 1913 passing yard and played half a season him going 4000 yards does not sound so crazy.
So with that in mind do you think he throws over 4000+ yards. I personally think he'll throw around 4200-4400 yards. If he can get his completion percentage to around 60% better I think it is certainly a possibility.