UPDATE (July 2019)
Since v4.0 (v4.1 here ), you can simply use either bootstrap-grid.css, or bootstrap-grid.min.css, it only includes the grid system ( v4.0 & v4.1 ) and the Flex utilities ( v4.0 and v4.1 )

Old answer:
At this point in Alpha, you can switch between true / false to use flexbox (when using a CSS preprocessor) according to the boot of V4 Alpha docs
Flexbox support finally came to Bootstrap. Join the new CSS layout styles with variable movement or sharing stylesheets.
How it works:
If you are familiar with changing variables in Sass or any other CSS preprocessor, you'll be right at home to switch to flexbox mode.
_variables.scss $enable-flex.false true.- , !
, Sass, CSS Bootstrap flexbox . .