We’re finding this is a thing in the gitter helproom. If you choose the bootstrap default in the settings its using 4.0 now.
Most of what you learned is the earlier/present version of 3.something.something.
You’ll need to go to Bootstrap site and grab the CDN url they recommend and swap it out… if indeed you have the 4 version of Bootstrap, are using CodePen, and are using some of the classes you learned here in fCC.