I don't own an iPhone, but it's my understanding that you may get the indication on an iPhone that it's charging from some controllers, but the iPhone has circuitry that rejects charging unless an acceptable level of current is available. Given the small current level from the RC, the iPhone doesn't charge, but gives an indication it is. If you observe the iPhone percentage level while flying with P3P (GL300A and GL300B early controllers) I think you'll find that it discharges while you fly, even though the iPhone says it's charging. I believe this changed with the GL300C which doesn't provide a charge line in USB, or charge indication on the iPhone. I assume it was removed because it was confusing too many people.
This is the reason I fly with an iPad, the larger battery lasts a long time.
BTW, there's no such model P4S. It's just P4.