There is not a lot of difference between them. It looks on the surface like an A3000 + better display. Certainly the polyphony, timbrality and the number of FX blocks are the same. You get CD burning driver support. I think you might get more FX types.
I'm not sure where you are getting your pricing info, or whether you juse made a typo but the A3000 is cheaper everywhere than the A4000.
In the UK you can pick one up for c.£500 whereas the cheapest A4000 I saw was £780.
That £280 difference is just about enough to get another 64MB of RAM and a ZIP 250 drive, both of which are pretty essential.
The downside to the A3000 is the lack of graphical display. Trying to find a loop point by ear/numbers is a bit tedious. You can do it on your PC but you'd need to invest in a cheapo SCSI card first. The you can use things like Wavelab which has amazing editing and looping facilities.