I dont see why you would upgrade your PA800 to a T4 if you allready have a PSR 900 in your setup?

The PA3X has some nice additions over the PA800,

8 vs 4 Multi effects
200 more sounds of very high quallity, espescially the new DNS and (E) piano sounds
80 more standard styles
300 more positions to save styles
The styles have 4 fills instead of 3, and the guitar mode in styles is hugely improved
Double the sample ram
The PA3X has a wonderfull chord sequencer...
The PA3X has a ribon controller and 8 assignable sliders
Very much improved vocaliser

On the downside it is a kg heavier and has no build in speakersystem... which in your situation dont really matter.

In the end, the PA3X is much more complete then the PA800 and sounds way better, the choice is yours tough.