Four days in Valladolid? Seems like a lot to me, so a day to Uxmal would be a grand adventure.
A few years ago I did a big loop that included Uxmal and Valladolid that I enjoyed very, very much, but it was a lot of driving. You will end up coming back to Valladolid in the dark, so I wouldn't recommend it for people unaccustomed to driving in the region.
Beginning from Valladolid around 8 a.m. head south on Hwy 295. Drive to Tihosuco and visit the big church in the town center where the back half has fallen to ruin and the chapel is open to the elements. Then visit the Guerra de la Castas (Caste War) museum. Go through it quickly unless you read Spanish as there are no exhibits in English and none of the guides (at least when I've visited) speak English. Be sure to do a little reading about the Caste War before you go.
Just past Tihosuco the highway splits into two 295s. Take the one to the right heading west to Saban, Tabasco, Dziuche. Stop briefly at Dziuche to see the biggest lake in Yucatan, Chicnancanab (Lake of Bitter Waters, I believe it means).
Turn north and head to Oxkutzcab where you will turn off the highway and take the Ruta Puuc route that will take you past numerous ruins but eventually get you to Uxmal.
From Uxmal, head north toward Merida until you get to the Pereferico (the main highway that circles the city. Head east and follow the signs to Cancun. Be sure to take the Cuota (toll road) back to Valladolid.
It's a long, long drive, but there is a lot to see and do, maybe more than you anticipated, so perhaps you might think about stopping for the night near Uxmal and continuing back to Valladolid the next day (taking back roads by Mayapan and other sites).
Have a great trip!