I took this from SnackyStacky's FAQs at the top of this board.
"5.) DOLLAR CREDITS: If your ticket has been used, and it's been MORE than 7 days since your past use, you can get a pro-rated dollar credit for the remaining admission. Once again, the pro-rated credit you receive is base on Disney's GATE price of the pass. This does, now, include broker passes. (Hotwire, Ticketmania...) That dollar credit is then applied toward new admission. The NEW admission must still follow the first two guidelines. "
The first two guideline are
"1.) Any ticket that you upgrade to must, at a minimum, have the exact same number of days and the same option configuration. That means if you want more days, or to add more options, you may. But you can't take away any options. For example: If you have an unused 4 day Park Hopper Pass, the bare minimum you would be REQUIRED to get would be a 4 day base with park-hopping and non-expiration. If you wanted to get more days, or get the magic plus pack option, you could add those. If you have a 5 day Park Hopper Plus Pass, at a bare minimum, you would be required to upgrade to a 5 day base, with premium and non-expiration options. You could add more days if you wanted to. This applies to ALL the transactions I'll describe below.
2.) In all transactions: If your Magic Your Way ticket costs more, you obviously will have to pay the difference. If your Magic Your Way ticket costs LESS, you will LOSE the difference. Disney won't give you the difference back!"
In your situation, you would get the credit based on the GATE price of 1 day to be applied towards the purchase of a 4 day pass. But why wouldn't you just buy a 10 day pass? It would cost less in the long run.