The fourth color recognized by AKC, and not FCI is "White" as in Color headed whites.
Me with Color headed white puppies - almost exactly 16 yrs ago, my first Collie (a pet) came from this litter, I was 12 yrs old.
FCI doesn't recognize sable merles, and they are not allowed to be bred. In MOST FCI countries breedings of Sable to blue merle is not allowed, or if its allowed, none of the sables (sable or sable merle) can be registered. Just as breedings a Rough to a smooth is also not allowed....
In the AKC standard, Sable merles are not listed as an recognized color, but many are shown and finished with no problems. There has actually been a HUGE debate among the Collie Club of America members as to whether sable merles should be added to the standard, since people breed, show and finish them anyway. CKC (Canada) does recognize sable merles though.