Yes, I have measured the difference in the RF power (control signal from RC up to craft) between P4 and P3 controllers, as well as the RF power (video downlink from craft to RC) between the P3 and P4 craft.
Here's the thread from a while ago. My conclusion is the GL300C controller is the weakest lightbridge controller DJI has manufactured to date. To really solve the P4 range problem it's best to start with a good controller, like the GL300A (early Phantom 3P), or the GL658A (early Inspire), they have much better transmit power. You might find them on Ebay used. However, you'll need to KEEP the P4 controller for firmware updates to the craft. These other controllers won't work for that.
This reduced power output from the GL300C versus GL300A/B has been confirmed in the FCC filing that DJI submitted to the FCC, as other members have reported.