I think it depends on the children. My DS would never sit in a stroller...yes, even in WDW....after he started walking at 11 months. We bought the stroller with us until he was about 2 1/2 and it was more of a pain than helpful. He wanted to push it instead of sit in it, and taking the stuff out of it every time we wanted to go into an attraction became a nuisance. The only thing it was good for was for carrying all our stuff through the airport.
If the 4 yr old uses a stroller at home all or most of the time, it might be worth it to bring. If not, I would just rent one when she really needs it.