It should be possible, but beware that locking or unlocking the bootloader wipes all the user data (for obvious security reasons). Boot your Pixel 4 in fastboot mode (Vol Down + Power), connect it to your PC with a known-good cable, and run the appropriate fastboot commands (
fastboot devices will show you the list of connected devices).
Since this effectively wipes user data, you might be better off running the installation process again from the web installer. This time, follow all the instructions and you should be good.