I want to keep the saved data and down loaded games on my hard-drive when I sell my PS4, so that when I buy a PS4 Slim I can just pop my PS4 hard-drive into the new one. Will any issues arise? Is there a different hard drive format for a PS4 Slim? Is it incompatible?
Based on this comment and answers on the PlayStation blog you CANNOT just swap the drives over. - The Hard drives are encrypted and keyed to the system they are in.
You can however back everything up to an external drive and then copy this data to the new system once you have it. full instruction on how to do this can be found in the PS4 user guide