I have a RP 4. 8gb. My SD card got erased. So I reinstalled the original boot using Raspberry Pi Imager. Now I want to use Ubuntu Mate as my OS. Do I leave the SD card in the RP4 and then have a USB with Ubuntu Mate? I then turn on the RP4 and it boots?
I got a little lost because I don't know if you mean Raspbian as "the original boot", but I don't think my confusion matters that much.
Since you apparently have not done much with your very-new installation, I would recommend you simply download a new Mate for Pi image from the ubuntu-mate web site, re-image the SD card, and then enjoy your new Ubuntu Mate Raspberry Pi 4 after you go through the installation steps.