I'd start with 3 then migrate to 4. Also, learn Flexbox as it will be enabled by default in BS4. FlexboxFroggy.com is a nice introduction. There's one more brea

I'd start with 3 then migrate to 4. Also, learn Flexbox as it will be enabled by default in BS4. FlexboxFroggy.com is a nice introduction. There's one more breakpoint in BS4. The css class to make images responsive has changed for the third time in 3 releases. Other than that, basically the same. I've been toying with it.