Things to in Freiburg im Breisgau with a 4 year-old

Europa-Park. Forget Disneyland, this is where Europeans go. https://www.europapark.de/en

Seepark Freiburg - large lake, very popular park.

Colombischlössle - Archeological museum with interactive displays for kids ranging from prehistoric to Bronze Age. It's really quite good. https://museen.freiburg.de/museen/arco

Wild animal park - https://www.mundenhof.de/

And, if you are over by the Augustiner Museum, look for the alligator in the river.

Buy a rubber duck or two and enjoy the Brächle running down some cobbled streets.