how could I keep my ps4 stuff? Just back it up to an external drive, trade it in, then reinstall on a pro?
If I could trade in my spare wii u and my current ps4 and get a pro for another $50 that wouldn't be a bad deal.
If you have super fast and data cap free internet, you can just backup all your saves to the cloud (or a usb drive), trade it in, and then restore the saves and redownload the games. By default your PS4 should be backing stuff up to the cloud with PS+ anyways. Mark all the games to install before you go to bed, put it to sleep, and when you wake up everything is back to normal.
You can also backup the entire system to a USB drive, takes a few hours though. Then restore on a new system from the drive, it restores everything including login settings, network settings, etc. If you have stuff like PT you pretty much have to do this.
The wii u is a tough and personal choice. I don't really play old games so I don't have any nostalgia for keeping old systems around, plus emulators seem to get better and better every year. I traded my wii u in last year and zero regrets, I'm excited for switch and 99% of the games from wii u I like I'm pretty sure will get switch ports, just like the majority of ps3/360 games have gotten remasters or BC on xbone. Of course the more esoteric games will probably not, like I imagine bayonetta or W101 will not get a switch port.