Is there a maximum distance when connecting the controller to the aircraft? I've been flying the P4 for ~ 3 weeks. I want to see what kind of distance I can achieve, but don't want to fly it out farther than I can see it. The terrain in my area doesn't allow me to have a great line of sight distance, so I did a test by placing my P4 in my backyard behind a secured door on my hot tub, turned it on and travelled about a mile away on top of a good sized hill. I couldn't connect up to the P4. When I returned the P4 was still powered up, but I still couldn't connect. After trying different scenarios I finally had to power down my tablet and back up before they linked together.

I'm running in manual on channel 20. Does this set both transmit and receive frequencies or just the return video and MC data?

The channel selection only selects the channel for transmitting video and telemetry from the drone back to the RC.

You were prevented by ground clutter and the door.  Your P4 will easily fly further than you can see,  so if that is to be your limit,  no worries.

The distance depends on the flight environment.Besides, please make sure the RC and antenna are in good position during the flight: