A precise numerical forecast is pointless, but be mindful all top 4 are already beyond the best tennis age. Statistically the most likely age to win a slam is 23, only a small fraction of slam titles are won after the age of 26.
Wwe may be close to the end of an era, even though it doesn't feel that way yet. I doubt Federer, Nadal, Djokovic and Murray will win another 10 slams combined.
Let's say they win less than 10 more slams combined. How about 8 or 9 more slams combined?
Federer: 1 more slam - 18 slams
Nadal: 2 more slams - 13 slams
Djokovic: 4 more slams - 10 slams
Murray: 2 more slams - 3 slams