1 Slam i think is guaranteed. He's still one of the best players out there and being in a paralel tour where he doesn't need to force his body too much, so at the right times he should be at the same level of his main opponents.
Also now he has nothing to prove and nothing to lose, so some of the chokes he did should be history now.
Federer will be a major contender in all Slams and i think he can win at least one of them.
The top 4 ranking at the end of the year is not even under question mark, the top 2 is, but not the top 4.
Yeah, 1 slam in about 3 years and another one is guaranteed.