sounds like a corrupted cloud save
i keep that setting off in the Uplay client .. it causes more trouble then its worth and prefer just to do manual backups of my saved games
here is a video about how to fix a corrupted save with Far Cry Primal .. should be about the same steps with far cry 4 i'd imagine
https://www.youtube.com/watch?v=ZIldEExOZ54
if ya need backed up save games i have some zipped up i can post (covers just about every mission of the game )
and has a read me .txt file included on how to use them
you will need to shut off cloud save feature in Uplay settings to use them
let me know if you need them and i can upload em in a zipped file and post a link on filedropper where you can download em from
good luck!