It needs to convert the data on that save file, not just send old data over.
The PS4 simply does not allow any actual PS3 content, which is why there has to be a separate system which is sort of like a "cloud" back-up/transfer.
The Meaning of Life, the Universe, and Everything.
Dont own a PS4 however this is what I would possibly consider trying.
If you have a PS4 running MC, you could export a game save and take a look at the file format/type You could then try to trick your PS4 by taking a PS3 save, exporting it, changing the file type and then uploading it onto your PS4. I am sure it wont be that "simple" but sometimes the easy things work best. As long as you can load the saved file and then save it as an actual PS4 file format you should theoretically be good to go.
Any kids that question my logic can keep this in mind: I'm a certified network tech with 9+ years of computer industry experience. This is just a small recommendation that MIGHT work.
Hammer, that's a good suggestion. However, I believe 4J studios utilize their own software to actually "convert" a ps3 map to ps4. Certain adjustment need to be made so that the map can successfully transfer