Bootstrap 4, add space between two card decks

There is no mt-20 in Bootstrap 4. The Bootstrap 4 margin classes are...

m{sides}-{size}

Where size is from 0-5, and size is a portion of the default spacer unit of 1rem

• 0 - eliminate the margin
• 1 - set the margin to .25rem
• 2 - set the margin to .5rem
• 3 - set the margin to 1rem
• 4 - set the margin to 1.5rem
• 5 - set the margin to 3rem

So you can use mt-3, mt-4, mt-5 etc..

http://www.codeply.com/go/29IGJHkqVd

You could put a bottom margin under the card class:

.card{
    margin-bottom: 10px;
}