Well, first of all it´s 9 slams, not 7 (4+2+2+1), so that´s a little over two years. That is quite a bit of time, especially if you consider there could be 1 or 2 surprise runs in this period. So, let´s say the summer of 2015 is when the top 4 has won these tournaments. Could the young guns beat them after that moment? I feel that that will be more probable than we can imagine right now. All of the top 4 will have rounded the dreaded cape of 600 matches then (they already have done so, save for Murray) after which players are said to deteriorate physically and it will show in their movement. They will lose a step or two and as we have seen, that can be enough to start a domino effect as far as one´s game is concerned. Meanwhile, the young guns will improve (just how much remains to be seen) and therefore, the distance between the top 4 and the rest that looks so huge right now might become surprisingly small by then. But let´s stretch that period a bit: by the end of 2015/beginning of 2016, every member of the top 4 will have won the bulk of their slams and will have ceased to be dominant, is my predicition. Only an incidental slam might be won by them afterwards.
Yes. I miscounted. But still, I think they have more years of domination (at least Murray and Djokovic). And even when the next generation steps up, they can still win a slam after that.