"Something happened on our end" is a bug some people are hitting that'll be fixed soon.
In the meantime you should be able to work around it by doing the following:
1) Navigate to the bundle page in the store for the bundle you ordered (standard edition preorder bundle or ultimate edition preorder bundle)
2) From the bundle page, do not go to the standalone Gears 4 page. Instead, go straight to "My Library" - you should be able to click Manage to do this
3) From "My library" click the download arrow next to gears of war 4