It was discussed in the YTY saves thread. You have to bring it to your PS4 using a copy of '14. Then save it as a '14 RTTS. Then on '15 use the option to import from a '14 save. '15 cannot convert the file when it brings it in from the cloud.
There are some guys that volunteered to help people do this in that thread or the other options were to buy '14 used from gamestop go home and convert it then bring '14 back for a refund, or get a free gamefly account have them send you '14 and do the same.
Go here to see discussion about this and other issues with YTY Saves