After hours on line with customer support, my feeling along it was the app issue was correct. Latest version of IOS and DJI Go 4 is a no go right now.
Grounded till app is updated

Lately, the latest iOS 11.2.5 might have the compatibility issue with DJI GO 3.1.24, 3.1.26 or DJI GO 4 4.2.4, we have an announcement about this. https://forum.dji.com/thread-131074-1-1.html
If your mobile device is running in this version, you can downgrade the iOS to 11.2.2 to solve the issue. Hope it helps.

DJI Thor Posted at 2018-1-26 19:40
Lately, the latest iOS 11.2.5 might have the compatibility issue with DJI GO 3.1.24, 3.1.26 or DJI GO 4 4.2.4, we have an announcement about this. https://forum.dji.com/thread-131074-1-1.html
If your mobile device is running in this version, you can downgrade the iOS to 11.2.2 to solve the issue. Hope it helps.

“Lately, the latest iOS 11.2.5 might have the compatibility issue with DJI GO 3.1.24, 3.1.26 or DJI GO 4 4.2.4,”

DJI-Thor,
With respect, I don’t believe that it is IOS which has a compatibility issue with DJO Go. It is DJI Go which has a compatibility issue with IOS.
IOS is probably the most prolific operating system on earth so Apple sets the standards for that environment. Apple makes betas of its IOS system releases available for any developer who wants it and they do that specifically to avoid the situation that DJI now finds itself in with DJI Go. It is up to DJI to ensure that it’s applications are compatible with Apple, Android, Windows etc and not the other way around.  The recent issues with firmware and this latest issue with DJI Go clearly shows that DJI does not undertake adequate testing of its products.

DJI Thor Posted at 2018-1-26 19:40
Lately, the latest iOS 11.2.5 might have the compatibility issue with DJI GO 3.1.24, 3.1.26 or DJI GO 4 4.2.4, we have an announcement about this. https://forum.dji.com/thread-131074-1-1.html
If your mobile device is running in this version, you can downgrade the iOS to 11.2.2 to solve the issue. Hope it helps.

I just posted under the DJI announcement on this. My P4 is flying OK with that iOS level and latest DJI GO 4. The notice states "Series 4 with HDMI module" which mine isn't.
I did note "Disconnected" on initially starting everything up, but after then power cycling the craft (with controller and app still on) they connected OK and I was able to fly.

I have an issue with Android 7 and the DJI Go 4 4.2.4 app, i cannot connect to AC

Hi all, DJI GO 4 4.2.5 is now releasing in stages, please check if you can update the app. The issue should be resolved now.