Given that they've been working on 4 for so long, I'd learn 3 now since it works and the migrate to 4 when it's ready. And for the record, I'm tired of everyone bashing Bootstrap. I'm using it for a production site and I love it. I'm a one-man shop and I need to earn revenue now. I can't afford to spend the next six months learning the quirks of CSS and its crummy layout techniques. Bootstrap has allowed me to create a responsive website that works well across all devices. It also looks much more professional than what I could have done on my own, not to mention the fact that my site looks much better than those of my competitors. I'm grateful that Bootstrap is around.