I've got an external for my PS4 Slim that isn't full but I lack another drive to do a full back-up of all my data.
I want to get a PS4 Pro, but I don't want to have to download anything again as my internet speed is shit.
According to this, you should be able to transfer everything with just an ethernet cable and being signed into PSN. No external HDD needed.
Ooh, this is a good pointer from them:
For the data transfer, only your new PS4 Pro will need to be connected to your TV. Sign in to the new console with your PSN ID and password to get started.
Yeah, except I plan on getting rid of my Slim PS4 first.
So, I guess I'll just transfer everything to my external and then restore it later.