Hello, Bill, and welcome to
planDisney!
Hakuna Matata, I’ve been in your situation plenty of times and I’m happy to share what my family has done in the past.
Option one, it sounds like you're visiting as 4 Guests, so
book a room that accommodates 4 Guests. If your reservation is a “room only” reservation, you can usually head down to the Front Desk a day or two before your extra member is joining you and add them to your room, so long as the room has the availability to accommodate the number of Guests staying. This past August my sister joined my Mouseketeers and I on days 3 and 4 of our visit. The Cast Member stated that there was no additional fee to add her to the reservation, so I just included her from the start. We did just have a room only reservation. Keep in mind when booking as a
vacation package, I do recall that everyone staying in the room must have the same features. For example if you all decide on a 4 day room and ticket package, everyone, including the Guest who can only be there for 2 days, will have the same 4 day ticket.
You might also want to consider a split stay. Depending on your party type, you can all start in a larger room and when the other member leaves, moving to a different resort or smaller room type. Bill, it all really depends on what you and your family want to enjoy. I know when I’ve condensed my visit, I struggle with leaving earlier than everyone and usually end up extending my stay. I don’t know if that's an option, but fingers crossed your family member can just stay!
I hope this helps give you an idea of what your options are. If you have any other questions about your reservation or options, I recommend giving the Walt Disney World Resort
a call. A Cast Member there will also be able to help make your
visit possible!
-Shantel