Main differences from what I can see between the two models is that the 428XD has the following:
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Subwoofer output: this KURO has high-grade audio parts: but if you want to boost lower frequencies, it has direct sub out, with no need for an additional amplifier.
ISF calibration ready: The ISF C3 (or Custom Calibration Configuration) feature allows your screen to be optimised for your viewing environment. As an optional service, a trained professional can precisely calibrate your plasma panel with customised levels of contrast, tint, colour level and other parameters.
However, no need to be baffled by all this science. Suffice to say that the picture on the 4280XD is breathtaking as it is on the 428XD and If anyone can see the difference......then I'm a Chinaman! Go for the 4280XD if you buy from
http://www.tlcbroadcast.co.uk/pioneer/index.php you can get it for £1,625.00 including a 5 Year Warranty