It's not as if Nishikori is losing solely to the top 4 guys. He can be brought down by a lot of players, so I just don't see him winning a slam. If Tsonga hasn't done it, I don't see him doing it. May be wrong but it'll be unlikely.
It just depends whether Nishikori can stay consistent, as well as being injury free. I'm pretty optimistic about his future after Tokyo, because if he can keep up that high level of play from now onwards, he will certainly be dangerous for years to come. He reminds me of Davydenko with that "playstation" sort of play. Who knows. He could be the next "Davydenko", but one who can win a slam one day.
Tsonga's just unlucky that he was born playing this Fedalovic era. He'd win slams for sure if he was playing in a future era.