However, without a device, you may not be able to take off in the first place. Lots of P4 owners tried, but could not, until the new P4 DJI GO app was released. However, once in the air, unless you are in a No FLy Zone, you should have full control without any device active, should your tablet crash or the battery fail, as long as the RC LED light is green. If it is red, you are SOL until RTH kicks in and brings it back within range, which will turn the RC LED to green again.
No - it will take off just fine without a mobile device.