My son is now 9. When he was 4 he had Perthes diagnosed on left side. It was vague for us too. It has to do with the fact that they are still growing and forming at that young of an age. It seems to me most doctors agree that at that point unless they exhibit certain signs there isn't much they can or should do. Don't be discouraged that they aren't doing anything but watching. My son went the whole time with his left hip with just going to the doctor every few months and getting x-rays. Watching the progression. It eventually began growing back to a more normal and he has no pain on that side anymore.
Now he did develop it on the right side at age 8. So this time because he is older and his bones aren't quite forming anymore we are being a little more aggressive. But not overtly so. He has been taken out of contact sports and such type of activities but is allowed to do what is comfortable beyond that. He wears a brace a night that keeps his legs apart so that hip is in the right place in the socket for it to heal. If you'd like to chat or anything you can email me at firstname.lastname@example.org