I'd learn Bootstrap 3 and then just migrate over to B4 later (if at all). The key is to learn the concepts... and those won't change much even if some details

I'd learn Bootstrap 3 and then just migrate over to B4 later (if at all). The key is to learn the concepts... and those won't change much even if some details do. I'm following this approach with Easele (https://www.easele.com/ https://www.easele.com/).