What Bootstrap Version is been used

Hello,
How current is the bootstrap version that is being used in FreedCodeCamp’s curriculum. Specifically, what version is it. Since there are have 2 dozen version of boostrap over the past decade, what versions are we required to know. When bootstrap is this way, it can often be frustrating for new beginners to catch up with every latest version.

I believe that the Bootstrap used by freeCodeCamp is all v4. Just worry about learning the newest version.