I have a PS3, and am thinking about getting the legendary edition. However, I am changing to ps4 soon, so I was wondering if I got it for my PS3, when I got ps4, would it be there to download?
No, each system has a separate game and both would need separate downloads. (Two separate purchases.) If you bought it on PS3, the option to download would show up on PS3 and the option to buy would show up on PS4.