I did a google search and it says it will work as long as it fits in the HDD bracket but it may not run at the top speed of the hard drive and could be slow.
Thanks garfield24, nice to see someone has manners. Others are simply saying that it is "unreal" for a PS4 hard drive to work with a PS3. Shows that they most likely don't even know what a hard drive is. So I will apologize to everyone, for their ignorance.