A four-year-old might think it would be fun to play guitar, but it wouldn't be. A four-year-old has small, soft immature hands. The connective tissue is nowhere near as hard as an adult's.

At best the kid will suffer frustration because she can't press the strings hard enough. At worst, she will suffer injury to her little hands.

Consider the kind of pressure and the concomitant pain that it takes to develop guitar calluses. Do you really want a little kid to endure that? Do you really think she would tolerate it?

Go to a good toy store and get something a child her age can enjoy. If she pouts, tell her that Santa Claus only gives guitars to kids age 12 and up.