Hi all,
I'm pleased to advise that Bootstrap4 has been implemented as part of our new template "Twenty One" in the upcoming v8.1 release.

Version 8.1 is currently undergoing public pre-release testing. Please use this opportunity to test it out and share your feedback with us: https://whmcs.community/forum/533-whmcs-81-beta-discussion-board/

Feature Documentation: https://docs.whmcs.com/New_Twenty-One_Client_Area_Theme

To learn more about the beta and to take part, visit https://beta.whms.com