For mine,,, it turned out that the patch was monstrous and required more drive space. I finally, painfully... uninstalled Fallout IV after all these years. That seems to have fixed the updating problem.
Same, the update originally was 1,8GB but it ended up using more than 70GB but steam said it only downloaded 19,5GB
Uninstalled the game and made three attempts at downloading and installing again, all stopping at around 55/59GB
On the last attempt, when the download paused due to lack of disk space, I restarted my PC and that magically freed up 70+GB and the last game data could be downloaded and installed.
So thats my fix for now. Wait for DL to pause, then restart and continue. Hopefully won't have to uninstall and reinstall the whole game every update.