Hi, having same problem too - from Bali, Indonesia. Phantom 4 got 17-20 sats. iPhone6, DJI Go4. Every time i try "Follow Me" it said "Satellite signal too weak" - not solving the issue in my iPhone - but I think I know what was the problem.
Since iOS updates, my iPhone6 itself having problem navigating Google Maps, Apple Maps - the GPS blue dot is not accurate and/or not stable.. keep moving in radius about 50-100m from my real location (nothing above me, straight to the open sky). I did reset network, settings. Also I did restore new setup from scratch. same problem.
So I highly suspect it was my phone GPS or iOS problem. I try using my old ASUS Zenfone (Android) test my P4, take off about 15m... bring up "Follow Me" mode... Satellite signal showing "Strong" and it work perfectly! same location like I flew before using my iPhone6.
Just share my experiences.
Always safe and responsible flight!