Hey Volks,
i started the Download for horizon 4 yesterday evening. I got back from work today, hoping i could enjoy the game und was finding myself looking at this. The Windows Store was saying, that the Download is 91/91 GB done. But that more than the game needs to download and the number just keep increasing. Whaaat is going on here?
Found the solution after neither Windows Tech or Xbox Tech support could fix it.
Open services.msc
Stop Windows Update service
Find SoftwareDistribution folder
Rename to SoftwareDistributionOLD
Restart Windows Update service
The download progress will go down to 0, then back up to around 35GB (mine was 35GB/69.50GB), and once complete it will install the game.
Enjoy!