If I purchase a 4 day ticket from Undercover Tourist, can I still upgrade in the parks to a 5th day?
Once you link the ticket from UT, it is no different than any other ticket. Which means that as long as you go to guest services before your final day, you should be allowed to add days.
The trick, of course, right now is whether you’ll be able to get a Park Pass for that additional day or not. So make sure you keep that in mind if you decide to add a day.