Unfortunately, there are no Bootstrap 4 themes. Even the github repository with bootstrap 3 was never fully functional and was full of bugs.
I believe that until cs-cart doesn't migrate it's main theme to bootstrap 4, it doesn't make any sense to bother with the custom bootstrap 4 theme development. To much hassle with upgrades :-/
As far as I know, cs-cart team doesn't have any plans to support modern front-end frameworks. That's a pity, because in my opinion cs-cart lacks modern themes which is the main factor taken into consideration while choosing e-commerce platform.
The more modern themes, the bigger community. Even though cs-cart is superior in many aspects, it looses many customers to all-shiny platforms :-/