I think what George, whose background information indicates he is quite young (I.e. 17 when he registered for the topic board) was trying to say is that $4,000 dollars is a lot of cash for someone his age.
George: I agree with you that some of the prices for Alembics currently out there are crazy....I recently saw a $2750 price listed for an Epic on eBay. These basses are overpriced and that's why they have been there a long time. When reasonably priced Alembics are listed they go quick so you'll need to move fast. On the other hand, if something is priced too good to be true....Look out! Personally, I'd hesitate to put down money on something like that unless I had the chance to play it first.
I'd agree with the others that if the Epic you found is in good condition and working order AND that's what you want in a bass, than $900 is a great deal...go for it!
If you want more in an Alembic bass, such a neck through construction, and a bit more with respect to controls, I've seen Persuaders and Spoilers listed for $1500-1800 and nice Distillates at around $2,000. I'm not sure where you are from but if you have transportation be willing to travel a bit to demo Alembics listed on Craiglist (bring a friend with you and be careful to meet in public places for any transactions).
The key is to be patient and make sure your getting what you want and need. Your young, so there will probably be a lot more Alembics in your life.