I can't read the thread to see if this has been mentioned yet (because I am just watching season 1 now), but Season 4.5 and a Complete Series set is being released on Blu-ray in July 2009.
I will probably now wait until I have the whole series on beautiful Blu-ray before I watch anymore of it!
http://www.highdefdigest.com/news/sh...r_Blu-ray/2720