To be honest I'd have to agree with several others that Murray really doesn't fit. He has been a great player, but I don't think he was up to the level of the other three. Personally I'd put it in the following order:
1. Fed
2. Novak
3. Rafa
They are all exceptional players, and each, without the other 2, could have achieved at least 15 more slams. However, Federer's attitude has always impressed me more than the others, so on a personal level I would put him at the top, as well as in terms of skill. I know that there are weaknesses in his record, but still you can't forget that he has got 20 GS. Rafa is an awesome player, but his record is inflated by 11 RG, with his next highest slam being the US Open, with only 3. Federer, on the other hand has only one RG, but the next fewest is five US Opens, so you can't say is dominance is Wimbledon only. Djokovic is also an amazing player, and has a great record, but though I would put him above Rafa, I don't think he's quite achieved Fed level yet. Maybe later, but not yet.