Very fine cracks around screws underneath the motors are common and can indicate serious problems hidden inside.
Or they can carry on with no problems and not get worse.
The P3/4K has same camera as P3Pro but reduced range, you can still expect up to 3000 feet and 1500 feet reliably. 
The camera gimbal and ribbon cable to it is vulnerable in even the slightest crash or hard landing. 

The camera on them is pretty good for video, and stills are nice too especially if you are familiar with using RAW.