428 has PIP, split screen, USB slot for photo viewing, extra 'hidden' picture functions and can be calibrated professionally, at a price. AND does not come with a stand.
4280 has none of the above but comes with a table top stand.
I have PDP-4280 XD . I will tell you now this is one of the best Tv Plasma screens i have ever seen If you see it in action your see what i mean , Very nice TV
Also it does come with Split Screen, Because i use it , Just saying that as aguy above you says it dont but it does !
Also if you hook it up to HD Full it looks Amazing . Might not be Full HD , But Trust me you cant Tell the Differance as it looks so dame good