Dedoimedo published a tutorial about fixing the issue of black border bars and wrong monitor resolution on Raspberry Pi 4.

RPI 4 & Ubuntu MATE - Full HD monitor shows as 1824x984 px

If there was a problem, yo I Tux it. As promised, a tutorial showing how to fix the issue of black border bars and wrong monitor resolution on Raspberry Pi 4 whereby 1920x1080px screen shows as 1824x984px by disabling the overscan boot configuration option. To wit.

RPI 4 & Ubuntu MATE - Full HD monitor shows as 1824x984 px