Ubuntu 19.10 server was recently released with official support for Raspberry Pi 4 SBC. Shortly after I read stories about the USB ports not working on the board, but it took another interesting turn as Canonical now explains the bug only affects RPI 4 with 4GB RAM, and USB works just fine on boards with just 1/2GB RAM.
The issue has been identified and it's been found to be a kernel bug with a solution in the works that being tested. In the meantime, you can access to your Raspberry Pi 4 4GB USB ports by limiting the memory