Just got my P4 Pro.  Of course it starts raining in Texas and will continue until Monday.  Time for setup.  I download the new app to my IPhone 6+, I charge battery and RC, I connect the phone to RC and power up.  I get the firmware upgrade for the bird and the RC unit and everything connects, camera, gimbal...  I power down and start back up and it says another firmware update required, so I get it and restart, now I only get "disconnected" in the app. (I installed the HDMI module easily with no issues).
So I check the site and try to link manually using the cst1, cst2, and record button, the RC goes blue and beeps so I hold the small light between the sd card and usb on the phantom and the RC stops beeping and goes green.  Still the app shows disconnected after several reboots, so I tried linking in the app and it goes nowhere.
I have until Monday to figure this out so any ideas?  I suppose I need to reinstall the firmware but I'm not finding a way yet.

Zombee grounded.

The 'disconnected' is usually an indication that the display device is not seeing the RC connection via the USB cable properly. If the light on the RC is green then that should mean it is talking to the aircraft OK (not always the case, but mostly so). C1+C2+Record is to relink the aircraft to RC, when the RC blinks blue you then need to push the 'link' button on the powered up aircraft (The 'link' button doubles up as the LED beside the SD card slot). When link button is pushed RC should go green indicating that all is well.
A quick fix for the 'disconnected' (Display not talking to RC) might be to unplug the USB cable and re-insert. Or, what I need to do at times is just power down the RC and then switch it on again (with display connected and powered up and DJI Go running).

Mine just did this a minute ago.  I've got a P4P+ & DJI GO 4 froze up this time.  It auto-restarted the app (remember, built-in screen version) but that didn't sit well with me.

So I turned off the drone and the R/C.  Powered the R/C then drone back up & voila!  Just a bad boot up, like any computer.  I was tapping away before it was finished booting, so that had to have caused it for me.

Aardvark Posted at 2016-12-2 16:13
The 'disconnected' is usually an indication that the display device is not seeing the RC connection  ...

Could it be the HDMI module and firmware?  I have been trying stuff with online chat and going nowhere.  I could put the old USB module in again if that wont mess something up.

ZOMBEE Posted at 2016-12-2 23:20
Could it be the HDMI module and firmware?  I have been trying stuff with online chat and going now ...

It could easily be caused by the HDMI module. If you cannot get the display to connect at all then I would try with the original module fitted. Have you tried another USB cable ?

When you connect the RC to the iPhone, do you see the message to 'trust this device'?  When you said that another update was required, was it for the RC or the aircraft?

DJI-Jamie Posted at 2016-12-2 19:35
When you connect the RC to the iPhone, do you see the message to 'trust this device'?  When you said ...

I believe it was for the aircraft.  I tried with dji support for hours, rolling back firmware, reinstalling the new go 4 app, but the RC firmware was not brought up.  I will try the old USB module tomorrow and hopefully wont have to return as they suggested.
I don't specifically recall seeing "trust" message, but I would have said yes of course, if so.

Update.  Got antsy.  Replaced original USB module and was ready to fly within seconds.  I have an HDMI module that should work fine for a P4, but I don't want it in the same room as my P4 pro.  If there is ever an update to allow this please let me know.
I could have saved hours of troubleshooting (I'm actually pretty good at this, now much better) if I would have ignored the HDMI module.  Now I have a Glyph headset and no way to FPV.  Any options here?

ZOMBEE Posted at 2016-12-3 11:06
Update.  Got antsy.  Replaced original USB module and was ready to fly within seconds.  I have an HD ...

It may have just been a bad HDMI module. You could try to get an exchange and see if the issue persists. That's if you really want to use the FPV goggles.