In some ways this has been the least exciting of his wins, but it is still cool. If he can avoid losing every match he plays for the rest of the year he'll have polished off what I think is important in the whole, "greatest of all time" thing; four solid years at the highest level.
There are problems with simple career totals (Alan Border is a better batsman than Don Bradman?), and other problems with just looking to averages (overly harsh on players who glide gently into retirement). A rough compromise would be to look at the best 4, 5 , 6 etc years of a players career. Four is a little on the short side but it's not far from "enough".
Each of Federer's four previous calendar years could be included in a debate for "best year ever".
Depending on how you look at it, it will be a very long time before die hard Laver fans will be compelled to give him up. Probably the same for Borg fans; the Sampras challenge is the only one that Fed will be able comprehensively demolish (though there will be those who witter away about how the competition Sampras face was so much tougher.)
At the end of the day though, there is probably no higher power in this discussion than the vague muttering of idiot pundits.