Prior to installing beta4, did you previously have a version of LinuxMCE installed on your system. If you did and you choose the option during the installation to keep your current configuration, then your login and password should be the same from the previous installation. Regardless, the behavior that your seeing is unusual.
When it reboots after the installation it starts well and it gets to the GUI with my user name displayed.
Are you referring to the Kubuntu login screen? When LinuxMCE is configured to start automatically, you shouldn't see this screen. Sounds like something went wrong with your installation.
You can try to log in to your system under maintenance mode by hitting the esc key during the boot process. You can attempt to use your password and username. If you get into the system, type startx. Hopefully kubuntu starts up. Go to the linuxmce control panel and click on the option to start automatically.