Alpha,
I'm not sure quite where you got this info, but I think you may have got your wires crossed a bit.
Channel 4 have only aired Seasons 1 and 2 of "Nip/Tuck". Season 2 ended in September/October 2006, at which point Channel 4 announced that they would no longer be buying any further seasons of this show, due to "low viewing figures"! (The fact that they kept changing the start times of the show, and sometimes changed the day it was on, for "Big Brother", of course, has nothing whatsoever to do with this!
Sky 1 had, by this time, already completed their transmission of most of Season 3, which dealt with the discovery of who "The Carver" really was. However, Sky have announced that no repeats were to be scheduled. Now, Season 4 is due to start in about one weeks time on Sky 1. (You may have already seen the trailers for it.)
As such, if you do want to see Seasons 2 or 3, then you'll need to buy or rent the boxed sets, in order to catch-up on the storylines, before you see Season 4, and you do need to see them, otherwise Season 4 isn't going to make a whole lot of sense to you, since new characters have been introduced in Season 3 which are pertinent to some of the Season 4 storylines.
Pooch