I know there have been several posts about this. I have tried several work arounds. Nothing seems to have done anything. But here goes, install keeps going past the initial download. Uninstalled, trying to reinstall again, still shows 64gb as the starting number on the download. After signing out then back in my download is now showing a size of 112.9 gb. So I’m now at 64.4 gb/112.9 This is strange to say the least. The 64Gb is not on my ssd. So IDK why it still shows that much is downloaded. Going to let this finish and see if it works. If not, going to attempt to get a refund. Was/am super excited for this game. Hopefully we get some feedback from developers in here soon. (Had this issue with the demo too, but I just paused it, and it the play button just appeared)
Managed to get my download size back to normal. Took a few tries with one of the work arounds in the pinned post about demo download issues. However, it now shows 28GB downloaded, but barely any progress on the download bar. IDK what to believe, the dowload size or the progress bar. Time wise the size makes sense. I had a “Forza Horizon 4 failed to install” message this morning. IDK why because the download is only about half way done. Gotta say this is the worst game buying experience I have ever had. Been gaming for 30 years. Never have I had to do more than press download for a download to occur properly.
Got my download done. What it took was ridiculous and not even possible for some. I had Microsoft support help me fix my download size issue, got the download working right. Had to go through MS Support, get some things done remotely by support… Anyway, after the fix was applied I tried to reinstall. Got most of the way done, progress bar resets, ruins the download, causes it to over download again, while not being able to install… The fix: Get MS Support to apply the same fix, and go to someone who has fast internet to complete the download before the Microsoft Store could mess up and fail the download. Going to someone’s house with fast internet is not going to be an option for everyone. The store and it’s issues really need fixing. Won’t be buying more games from there until this is addressed. So maybe never, who knows. 
I have the exact same problem and im so fricking pissed, i have played fh4 for like 4 days now and when i exited the game just to launch it again it said “cannot open because game is updating” or something and when i went to the page again it said something like the same as for you “Forza Horizon 4 failed to install” it said repair or reinstall and i couldnt find how to repair it so i reinstalled it and it didnt work for like 5 tries and then finally when it works and it downloads the game now it was somewhere around 50GB and i go to the store page to play it and it says “install” and i click install and it takes like 5 sec to install nothing i guess and it says “installed” and when i go back and into the fh4 page again it says “install” and i click install it takes 5 sec and i go back and into the page and the same thing over again
Also im using xbox game pass btw
Also when i search for forza in the store i see fh4 standard edition, ultimate and deluxe but after that and some forza motorsport titles i see “Forza Horizon 4” and it says installed but when i click on it and it takes me to the page it says “install”