Hi Brian!
I love vacationing in Walt Disney World during December too. There is nothing more
MAGICAL than being immersed in holiday decorations and festivities. Mmmm ~ I can smell the gingerbread houses from here!
If your no expiration tickets have not been used before, then you can upgrade and add more days on them when you visit in December. In order to upgrade theme park tickets, there must be at least one remaining day left on them AND they must be upgraded within 14 days of their
first use.
Here is a link to explain
Walt Disney World theme park ticket upgrade FAQ.
Since you described your park tickets as having "3 or 4 day left on it", I suspect you are not within your 14 day window of first use. In this case you will need to purchase new separate tickets if you wish to visit the theme parks for more days.
I hope this information helps with your vacation planning Brian. If you have any more questions, please come back and ask them here at the Disney Parks Mom Panel. We love to help!
Blessings,
Brenda ºOº