Infact, apart from AGA there is really nothing of an upgrade.
AGA is pretty crud, its slow, and you can do 256 colors with OCS.
I think AGA is fast enough

[youtube]WrSQPqtNPNA[/youtube]
I agree with you though, keep the A2000, you've only got it for nostalgic reason so if you're happy with it then you don't need to change it.
If you buy this A4000 it might turn out to have faults which will be a hassle, better to stick with what you know.