There is a small exhibit in the horse barn at Fort Wilderness on Walt and horses. Typically she can watch them groom horses there, learn from the farrier as he shoes horses, and either do the trail ride or pony ride depending on her age. Since the petting zoo has closed, they now have the ponies in the petting zoo area so she can talk to them. It's well worth the short boat ride from MK. (and if I'm hanging around the horse barn, I often end up teaching the kids something!)
Don't forget to check out the washrooms next to Guest Relations at Fort Wilderness as they are horse themed.
If it's a relatively quiet day, stop at the MK at the top of Main Street by the train station. When the horse drawn street car stops, the driver may allow her to pet the horse and teach her a little.
At age four she will probably enjoy the carousel at MK.
At AK, there are zebra to be seen on the Safari ride. You can get a better view depending on time of day from AKL.
At MGM, the only horse which comes to mind is in the Western scene in Great Movie Ride, which is quite scary.
At Epcot I can't recall any horses.
You don't mention where you are staying, but you should consider POR as there are horse drawn carriage rides and again she can see and learn about the horses there.
Hope that helps - I have never lost my love of horses and always detour to Fort Wilderness. It makes for a nice break during a busy day and is definitely worth it for anyone horse crazy....