I hear you,
@DarthFed, but the margin of error for Rafa is miniscule - just one RG loss and his chances of surpassing Fed become virtually non-existent. There are just too many things that have to go right for Rafa to win more Slams than Roger:
1. Roger has to stop winning Slams.
2. Rafa has to stay healthy for 3+ years.
3. Novak can't come back strong.
4. No young player has to rise to significantly challenge on clay.
Again it isn't only about being four behind - it is being four behind and Roger just won a Slam a week ago, and could win more.
But let's re-calibrate after RG. If Rafa wins it, he's still in the running. If he loses, he almost certainly won't do it.
Of course if Rafa wins RG and the USO again and, say, Cilic wins Wimbledon, then we're at 20 and 18 going into 2019, and the lead doesn't look so good...