4280 makes do with out picture in picture, USB home gallery function and a sensor which will automatically adjust the picture to suit the viewing conditions, not that I'd recommend using this. It also lacks the ISF C3 memories and has an inferior remote, not as plush. It does come with the pedestal stand which is a £100 extra on the 428. You'll have to decide for yourself how important the extras are, the screens themsevles are identical.
As for where to buy, I'd recommend a friendly local dealer. You can find your nearest one on
Pioneers website.
And personally I think the picture on these screens is fantastic, but then I'm biased