Another good project's source code to learn from is Skeleton. (more simple than bootstrap)
CSS version: https://github.com/dhg/Skeleton
Sass version: https://github.com/WhatsNewSaes/Skeleton-Sass
Edit: For more of a tutorial approach, check out http://learn.shayhowe.com/html-css/
So yes, you should learn CSS, at least so you understand how web technologies work, in general. But not learning/using Bootstrap (as the parent recommends), if you think that will make your life easier, seems silly. I recently had to do a couple frontend projects for the first time in said over-a-decade-ago, and I found Bootstrap easy to pick up (it took like a half hour), and made things a lot easier than rolling my own CSS, for very little cost.
To actually answer the OP's question, it depends on what you need to do. If you're starting new projects, just learn BS4. I did that back in February, and it worked out well for me. If you have to deal with some existing projects that use BS3, you'll of course need to learn that. But they're so easy to use, that if you already understand how HTML and CSS work (if you don't, of course that should be your step 1), you don't need to really "learn" it so much as read through the section on the layout model, and start building a page/site/app, looking at the reference docs as you go along when you want to add a dropdown or navbar or something.
I came across this when I was looking to gain a more complete mental model of how CSS layout works, as opposed to to the odd assortment of tricks, approaches, and googling I had been relying thus far.
The changes to normalize are listed here (Skeleton includes v3.0.2): https://github.com/necolas/normalize.css/blob/master/CHANGEL...