From what I've read in the car mags the MkV is due to be on sale in Feb of next year, with the GTi models being 'some of the first to arrive'.
Therefore I would expect them to be available on 04 plate from March.
Whilst a MkIV will indeed become cheaper when the MkV's are released they will also loose money much, much quicker, so you will have to do some maths - is saving a couple of grand at the point of buying gonna be worth it if that money and some is gonna be lost in worse residuals by the time you come to change again?
Personally, I would rather spend ?18k and drive round in a new car than spend maybe ?15.5k driving round in a car that had been replaced. That ?2.5K will still be 'in' the mkV when you come to change, and if the mags are to be believed you'll have a much more entertaining period of ownership with a MkV GTi than a Mk4 (its meant to be much better dynamically.