If I was buying a mobile device to use with the P4, it would be an Apple iOS device mainly becase they are far more likely to work seamlessly with DJI drones and software, including most 3rd party software. I've been using everything from my iPhone 6s to my iPad Mini to my Ipad Air, to my iPad Pro with my Phantom 4. All work flawlessly except for my old original iPad Mini, which choked on the video data unless I dropped the preview spec to bare bones. I found the iPad Mini to be the ideal size, so I picked up an iPad Mini 4 with wifi and cellular, refurb from the Apple Store for $429 and put it on my AT&T account for $10/month. That's works great for me.
If you're solidly into Android, the nVidia Shield get good reviews as long as it doesn't catch fire.