Upon starting i am getting message in yellow:
Compass redundancy switch

Which disappears also as quickly as t came.
Only happened once
Anybody knows what it means?

That's a rare and new one.  Anything out ordinary  happening?

Have you gone in to the settings to see if anything is showing red or has an option to switch over to avoid the error?
There are two compasses, as I have read here in the forum one is in the UAV and the other is from your mobile device.  Now I now many mobile devices have compasses, but to me that doesn't make any sense, the P4P has two IMUs I would think logically it would have two compasses too, but I don't know this to be fact.
If the UAV does have two compasses then that error seems all the more curious, as they are redundant.  What it might mean is there is a problem in the settings or there was a problem with one of the compasses that required it to switch to the other.
I wish the manual would fully explain what all these errors mean.
If you don't know, you can go in to general settings in GO 4 and near the bottom you can look at all the warnings listed by date and you will have time to see what they all were, as I agree they can come and go way too fast.

No nothing unusual. Nothing red. It didnot show up anymore. Just to be sure i calibrated compass and its flying fine

This means that your P4P changes from example compass no. 1 to compass no. 2 or vice versa. Most probably there has been some magnetic field near one of your compasses, which, after what I've been told, is sitting in each leg on the P4P. If the error is not displayed anymore, all is fine.

Regards Leif.

Landbo Posted at 2017-9-17 06:41
This means that your P4P changes from example compass no. 1 to compass no. 2 or vice versa. Most probably there has been some magnetic field near one of your compasses, which, after what I've been told, is sitting in each leg on the P4P. If the error is not displayed anymore, all is fine.

Regards Leif.

I agree with this response. Sometimes, you fly over an area that has magnetic interference and this message can come on. Soils vary in their iron content and this can affect flight if you near ground.

Phantom 4 Pro has two IMUs and two compasses. The message will pop up when the two compasses switched. As long as one set works well, the drone could fly. No worries.