When i turn on my phantom 4, should i also turn off cellular data on my phone? Or just put it into airplane mode so i dont get texts or calls when i am flying?

it does not matter. I've tried to talk phone but I also used it to fly with DJI go App ...
but today I use a tablet when I fly. (Nvidia shield k1)

Depends on you, if you dont want to be disturbed while you fly or not

It would be better if you put it into airplane mode, because I assume most people don't really want to receive a call or read text message when flying a drone.  It would be better for the signal reception of the RC.

I always set the tablet to airplane mode, and even my phone i turn off WLAN and ask friends around me to do the same, if possible.

One problem that might crop up with airplane mode is that you lose the ability to load maps.   Although, as a workaround you can pre-load the maps for the area you will be flying.

I use a cellular capable tablet.  I leave the cellular running for the maps but I always shut off WiFi.  (WiFi runs at the same frequency as the controller and might cause interference).  It seems to help a bit with the range.