Logitech 4000? Or do you mean the G400?
Both decent mice, the G400 is the spiritual successor to the MX-518, widely regarded as one of the best gaming mice ever and at £35 is hard to argue with, but £20 for the M8000x is excellent.
Having had the MX518 and M6800 (same shape as the M8000x but with less features) I much prefer the M6800, even though it's about 1/4 of the price and has less features. It fits the hand better and the buttons are better placed with a nicer feel. The G400's buttons are just a little too far back for fast, accurate use in games, although they're fine for web browsing etc.
I'd take the M8000x, there's no point paying £15 more for a very similar mouse.
