I saw in the release thread of BS4 that popper.js is not included.
My testing came up with finding that the localized version of Bootstrap.js v4 is the bundled version that includes popper.js functionality when you export. However, when you switch to use CDN in the settings, the version of Bootstrap.js that is used is not the bundled version so it does not contain the necessary popper.js functionality for things like modals, dropdowns etc...
So I would think the CDN version used during exporting should either be the bundled version or also include the popper.js in the proper JS order.
Saj