So I own fallout 4 on Xbox and I use Xbox cloud gaming to play it on my Ally as they will...

If you play connected to xbox live you game data will automatically be saved to the cloud. Important to note that the cloud is used only to save game data not the game itself.

Example: if you are playing Fallout 4 on your console and save your game and go to another console with Fallout 4 installed you can sign in with your xbox live profile and play from where you left off on the previous console. But you cannot save Fallout
4 the game to the cloud to play on both consoles without installing it onto there hard drives.

I hope this helps

I had the syncing thing when I was playing Starfield on my Xbox and ROG Ally, weirdly it was always when going from my Ally to my Xbox, not the other way around.

I eventually worked out what the issue was, I was switching off the Ally as soon as I stopped playing. What's I should have done was leave the Ally on for half an hour or so while it uploaded the save file to the cloud..

Perhaps that's the issue with you, the game knows there is an updated file, but cant access it because it wasn't uploaded fully. Perhaps turn on your old PC and let the save file upload.

Xbox cloud content would be shared with the PC if you're playing an
Xbox Play Anywhere game.

Fallout 4 is unfortunately not one of these titles, so your Xbox saves are not transferrable to the PC version of FO4 (or vice versa).

Yes I’ve cleared over 100 gb on my Xbox but xcloud still won’t work for fallout 4 and it’s only fallout 4 that has a problem. I tried using cloud gaming on my Xbox instead but it still didn’t work. My game starts syncing and then says “we can’t save any more data for this game or app” and says my Xbox is out of space for saved data

The PC version is a separate game