You cant ask for a certain amount of credits. What ever the bids are at the end of the auction is what you have to accept. Now if you mean, that if credits reach 4000 then you will ship for free, then thats ok. Also, it would be great if you told us a little more about it...like what brand it is, how many gigs it has, how old it is, and why you are listing it.
You can "back out" once you place bids, you have to wait to get outbid. That is why it is important to read through the description and ask any questions BEFORE you place a bid.