does he also bring his hands put to his chest? There is a disease that is genetic(not to scare you) It is called "Charcot-Marie-Tooth Disease" (Shar-Ko)
My son, who is also 4, has been walking on his tip-toes on and off since he was 2. It first became something that was just fun, then it became a habit. We talked to his doctor, who then sent us to physical therapy because one of his tendons was tight. She showed us a few stretches to help the tendon. Now, with numerous reminders, only one PT appt, he just does it once in awhile. The doctor and the PT said it is sometihng they just grow out of. Just make sure it's not effecting those achillies tendons.