Is the d4 better at focusing than the d800?

Nonsense. The manual obviously only states that the D800 is capable of locking focus at the highest fps achievable by that camera. That does not mean that the D800´s AF can't be faster still (like the one in the D4). In other words, how long it takes for the camera to lock focus isn't stated; just that it is fast enough to cope with the camera's fps.

If the D800's AF were fast enough to cope with 8 or 10fps (which the motor drive can't do), it would still obviously be fast enough for 5fps too (and the latter is what Nikon sensibly states in the manual).

Marco