4 days ago, huh. Have you sent a friendly reminder? Most sellers send a friendly reminder at the end of day 3, and then file an Unpaid item case at the end of day 4 (hour 96 if you're counting). Some sellers will wait a week before firing up the canon, but no reason you should. The following link will tell you all about what to do:
http://pages.ebay.ca/help/sell/unpaid-items.html
Once you open the dispute, the buyer has 4 days further to pay, and if nothing, you close the dispute, relist your item, get your fees back, no one leaves feedback, and the buyer gets a strike. You then block this buyer here:
http://pages.ebay.ca/help/sell/manage_bidders_ov.html#block
You then set your preferences to block all people with Unpaid strikes here:
http://pages.ebay.ca/help/sell/manage_bidders_ov.html#set
Some folks will tell you to block people without paypal accounts, but they don't do it themselves because they don't want to miss a sale. A great portion of ebay sales are credit card purchases. If, however, you are overrun with zero-feedback nonpaying newbies it may be something to try for a while. Some categories are more prone to attract irresponsible kids than others. You experiement as you go.
For now, though, if it is hour 96, get that Unpaid case underway and get those fees back.
