Although the GL658 works with P4 much better than the stock GL300C, and I highly recommend it for that, you may want to keep the GL300C for updating the craft, if you're the type to want to update with new firmware. Although the GL658 won't suffice to update your craft firmware, you can do that with digital assistant 2 if you don't have the GL300C controller. I recommending not updating, unless there's a compelling reason to. In the case of pairing a GL658 to a P4, the vintage of firmware for the controller and the craft "may" be out of sync if you run into issues, but frankly I don't think you'll have any issues. If you do, update both to the most recent firmware and that will work. If you can keep old firmware, before June 2017 on your craft, there are other freedoms you can open the door to later.
@Dacon Productions is right, the stock P4 controller GL300C is a POS, the
weakest controller DJI has made to date.