It looks like assets/styles/components/_grid.scss contains the only conflicts with upgrading from bootstrap#v4.0.0-alpha.4 to bootstrap#4.0 in Sage 8.5, but I’d love to get some confirmation on that from the Roots community.
Maybe I had installed an older version of Sage because 8.5.4 does have Bootstrap 4 (not alpha).
My repo had the alpha version. Then when I bower uninstall bootstrap and bower install #bootstrap4.0, bower stopped injecting bootstrap into main.scss.
Not sure why. Solution was to clone Sage 8.5.0 with composer and copy the changed files over.