My 4 year old daughter's feet ache almost everyday. She starts complaining of leg pain if she walks for 20 to 30 minutes. What should I do?

Not normal. Warrants a doctors visit. Does she have flatfeet?
Flat feet. I immediately think of flat feet as the cause. Flat feet are normal in kids but should improve by age 4. If she wears sandals or shoes without arch support the pain may be more obvious. Have your child seen by your pediatrician to see if this is the cause. If it is not, then referal to foot specialist may be advisable as it is not "normal" for a child to complaint of foot pain as yours is.
Apohysitis/Flat feet. Ask your daughter where it hurts her exactly. If she points to the bottom or back of her heel, then she has wants known as severs disease. This is common in kids and most people chalk it up to "growing pain". However, it is very painful to the kid. Usually an orthotic device will help. X-rays will also show inflammation as well. There are other growth plates that inflame in feet as well. C a dpm.
See Physician. Podiatrist and pediatrician is a good start to determine why this is occurring. Get it checked out.