Seems like everything is harder than it needs to be. Using Bootstrap 4 requires Popper.js for things like Tool Tips and Dropdown menus to work.
Bootstraps own site is less than helpful when it comes to building your own code, to be fair they are still in Beta. But Popper’s own site is maybe less helpful than that.
After spending about 30 min trying to get Codekit to properly minimize Bootstrap 4 with Popper I realized that the file
/dist/popper.js is an ES6 version…
I had to then include the Universal Module Definition or UMD version of Popper.js in order to have Code kit correctly Uglify the JS.