Change a 4 day pass into a 5 day pass on day 3
I read somewhere that, if you just began using the pass within the past 7 days, you pay the difference between a 4 day and a 5 day pass, nine dollars. Since you are inside the park on the third day, your 4 day pass already has 3 days ticked off, you will actually receive a "used" 5 day pass that already has 3 days ticked off. I could be mistaken but this forumula makes sense. If you changed the 4 day pass into a 5 day pass before going in on the first day, you would pay just the difference. Disney does let guests change their mind on a few things within a few days after the fact.
The normal formula is this:
Start with how many days are left. The pro-rated value of the used ticket is based on the purchase price. After entering the park on day 3 with a 4 day ticket, there is one day left so the value is 1/4'th of what you paid.
Next figure out how many days you want to end up with. You are looking forward to a fourth and fifth day so if the old pass vanished now, you need a 2 day pass.
So you are going to need to trade the remains of your old pass towards a 2 day pass. Incidentally a 2 day expiring pass without hopping costs about $127. including the tax.
With both formulas the best bargain is had if you trade up to a 10 day non-expiring pass.
Disney hints:
http://members.aol.com/ajaynejr/disney.htm