The topic says THE top 4 (I did understood it though to be CURRENT top 4).
Well, I can easily say 4 more years.... Let's read the name aloud (Just to assure ourselves)
And for a second, try to run the past 8 years (Yes, entire 8 years within a second) and try to imagine a tennis world without them. I'm not exaggerating here.
Nadal is almost a lock for two more FO
Djokovic or Murray for 2 more USO and 2 more Wimbly
Probably AO can spring up something but the field will be fresh at the beginning of the year, so it might be difficult.
I can certainly accept that someone else will come up but for your question of winning atleast 2 per year by the top 4 - yes that will happen for 4 more years, meaning till Rio and I'm expecting Federer to retire at the end of 2016.
If Federer's resume has holes, then everyone else's has more and bigger ones.