Adding this answer because the comments in the accepted answer are getting too long and it's not complete. As already explained, the hidden-* classes no longer exist in Bootstrap 4 beta.
"What exactly is hidden-sm-DOWN?"
It no longer exists in beta, but it previous versions it meant "hidden on small and down". Meaning, hidden on xs and sm, but otherwise visible.
If you want to hide an element on specific tiers (breakpoints) in Bootstrap 4, use the d-* display classes accordingly. Remember xs is the default (always implied) breakpoint, unless overridden by a larger breakpoint. Since xs is implied, you no longer use the -xs- infix. For example, it's not d-xs-none, it's simply d-none.
https://www.codeply.com/go/bRlHp8MxtJ
- hidden-xs-down=- d-none d-sm-block
- hidden-sm-down=- d-none d-md-block
- hidden-md-down=- d-none d-lg-block
- hidden-lg-down=- d-none d-xl-block
- hidden-xl-down=- d-none(same as- hidden)
- hidden-xs-up=- d-none(same as- hidden)
- hidden-sm-up=- d-sm-none
- hidden-md-up=- d-md-none
- hidden-lg-up=- d-lg-none
- hidden-xl-up=- d-xl-none
- hidden-xs(only) =- d-none d-sm-block(same as- hidden-xs-down)
- hidden-sm(only) =- d-block d-sm-none d-md-block
- hidden-md(only) =- d-block d-md-none d-lg-block
- hidden-lg(only) =- d-block d-lg-none d-xl-block
- hidden-xl(only) =- d-block d-xl-none
- visible-xs(only) =- d-block d-sm-none
- visible-sm(only) =- d-none d-sm-block d-md-none
- visible-md(only) =- d-none d-md-block d-lg-none
- visible-lg(only) =- d-none d-lg-block d-xl-none
- visible-xl(only) =- d-none d-xl-block
Demo of all hidden / visible classes in Bootstrap 4 beta
Also note that d-*-block can be replaced with d-*-inline, d-*-flex, etc.. depending on the display type of the element. More on the display classes in beta
Also see:
Bootstrap 4 hidden-X-(down/up) class not working
Missing visible-** and hidden-** in Bootstrap v4