Always getting IMU Error, Calibrate required on start up.  IT goes away eventually.  BUT IMU takes a long time to warm up!  Ive done a cold calibration (fridge) and that didnt help either.  Takes me like 3+ minutes to get in the air after start up.  
My mavic doesnt do this to me.  Why does the P4 move like molasis?
Phantom 4
Current firmware
iPad Mini2 iOS 11
DJI Go4 current update

The IMU error is a known problem which I thought was fixed in one of the most recent GO updates, but obviously not. Ignore it - unless the status continues to report that one is necessary after the P4 has warmed up.
The current firmware has issues which DJI have acknowledged. They are working on an update, although ETA is unknown.
I note you have an iPad Mini 2. Expect video transmission break-up when flying your P4. The CPU is under-powered to handle the video decoding load with the 4.1.x series of GO. You'll need a A8 or higher CPU for faultless in-flight images. The Mini 2 doesn't have this problem running Litchi, and it may be a cheaper investment than an iPad upgrade.

indeed, I have noticed crap video quality when flying my p4 lately..  

Ken448 Posted at 2017-9-28 19:12
indeed, I have noticed crap video quality when flying my p4 lately..

Don't worry about it, the video recorded to the SD card in the aircraft will be fine.

You can try to leave the drone for a while after powering it on to see if the error message will disappear. You do need to do an IMU calibration if it persists. For your case, I would suggest that you refresh the firmware once again.  

I had this problem with the latest Firmware. It went away after I did the Compass Calibration, now it boots up with out any IMU warnings.

beeper Posted at 2017-9-29 04:04
I had this problem with the latest Firmware. It went away after I did the Compass Calibration, now it boots up with out any IMU warnings.

hmm  thats strange.  Ill give that a go today