To answer this question, I'm more interested in how do you learn it first. All these frameworks are just COPY & PASTE the pre-made code. For example, you wanna button in that style: http://v4-alpha.getbootstrap.com/components/buttons/ Copy that code and paste into your HTML. Whatever v3 or v4 are the same way. If your learning way is memorizing the code without checking the document each time, then I'd say v3.
Copying and pasting wouldn't work, because many times you need to customize things. Also, what a nightmare would be to look it up every time you need to use a component! Of course learning it means to learn components by heart, just like you don't look up built-in functions every time when you use a programming language.
Take Bootstrap as an example, I open the following doc: http://v4-alpha.getbootstrap.com/components/alerts/
If I want a Modal, I will check Modal doc (accurately, I jump to CODE PATTERN and copy it, not starting to read doc explanation.) and use it. After few times, you will memorize the basic by heart. (understands the basic, then you can create your snippets or something. Myself, I still open the doc to see the visual examples. Anyway, I coded with a lot of frameworks in different projects at the same time, not just bound to Bootstrap. You need to find your best workflow yourself)
EDITED: "customize things" seems not suitable to the questioner at the moment. If he understands basic CSS, he may not ask this question. :)