Other than getting an offline job I don't think there are any ways to earn that much from a standing start, maybe if you had a few months of time to invest you could get to that point.
LOL what kind of question is this. There are millions of ways to make that much money, just get out there and go do it.
400 in 4 days start to finish is pretty far fetched man. There are ways, but this is the wrong forum to find them.
yes sell something on ebay, hmmmm I dunno 4 things worth a hundred dollars over the course of 4 days.... Easy! Come on man, what are you really asking??
Get resell rights to a product or software and the sales page or use PLR and put your paypal button on the page, charge what you think it is worth. Promote the hell out of it, by getting traffic to it in a number of different ways. I'm not going to get into all of the traffic strategies because I would be talking for 12 hours. That is a good and quick way to get money into your paypal account, if you don't have other assets to promote.
I agree, if you have some kind of talent, you can offer your services. Just be ready to put some major work in, to make that much, in that short of time.
You could write a ebook explaining how your going to make $400 selling it for $10, No review copy available. Thats a win imo.