On the "sour grapes" scale, I don't rate it too high, perhaps, 2 or 3 out of 10.
From the article, (Federer)
"So it shows that it is more possible today. But the feat, still regardless of the conditions are easier, is still and would be an unbelievable one. Credit to him."
If Fed said that he (and Rafa, as well) didn't have the same opportunity that Novak did due to greater homogenization of surfaces, that would be sour grapes.
I think he was praising what Nole did - and is trying for again - but also saying (what many agree with) that this would have been tougher to achieve back in, for example, Borg's era. That point is slightly debatable, but I don't fault Fed for bringing it up...and we don't know what question he may have been responding to.
To me, 4 in a row is huge.
In the Open Era, only Laver and Novak have been able to pull it off.
On the one hand, I rate Laver's 1969 a little higher than Novak's 2015-16 because he did it all in one calendar year.
This is balanced out, in my mind, by the tour being much deeper in Djokovic's time.
One can debate how much of a factor surface variation is. I tend to minimize it, as we're really just debating amount of variation. In either case, you still have to defeat the entire field (barring injuries) in the four most important tourneys of the year. It's not like players arrive there, and thn they pick the playing surface randomly.