As has been said there is a great deal of difference between the film and the actual stage musical. I have heard that there is a non-nude version of the play out there, but I think all the musical numbers are still intact however.
You realize, of course, that Porgy and Bess is filled with references to sex, drugs, criminal activity, adultery, and racist stereotypes, right?
But I’m only teasing you; none of that content will likely be evident, or mean anything, to a child of four. My kids loved Gershwin when they were young. And also Jesus Christ Superstar, 1776, and Cats (well, my daughter, anyway; my son didn’t care much for Cats). But what they liked wasn’t the plot or underlying theme in any of those; it was the music. Of course, you can only go to whatever shows are being staged. Your daughter might very well enjoy Hair, and in my opinion it wouldn’t curdle her brain or destroy her life, but it might be embarrassing for you if she really liked some of the songs and started singing them in public. Rattling off a list of racist epithets, no matter how catchy, is likely to cause her problems in school.
“There’s a boat that’s leavin’ soon for New York. . . .”