I took inspiration from these custom Foundation 6 cards, polished and reworked them to Bootstrap 4 Beta.
Card minimum height is set to 450px and will extend to adapt to text, buttons, etc... whatever you put in card body. Use Bootstrap card-decks to align them nicely in a row. If you do not want the card to expand, set a max-height or height atribute on card level and scrollbar will be displayed in the card body.
You can use these custom cards together with standard Bootstrap 4 cards or use standard card options, like colors, footers etc.
There is also a wide variant, without the upper picture cut and with centered avatar.
Small card:
Wide card: