Computer not connecting to my account

The BOINC client on each computer 'knows' what account it belongs to through a file named account_einstein.phys.uwm.edu.xml located in the BOINC data directory (which may be hidden on Windows - I don't use Windows so I don't know for sure).  It sounds like that file might be missing or corrupt on your latest machine.  It might even be that you have managed to create a 'different' account on the 4th machine.  Are you sure you used exactly the same email address on the 4th machine?  In any case, you could see if you can find the account file and browse its contents.  It should be identical across all 4 machines.

The first couple of lines are the important stuff, the master_url of the project and the authenticator which allows that machine to access your account.  Please don't post your authenticator.  The easiest way to resolve things might well be to delete that file on the 4th machine and replace it with a copy from one of your "good' machines.  Then try restarting BOINC on the 4th machine.  If that doesn't solve things, you could try re-installing BOINC, making sure you have a good copy of the account file already in place and that you get the installer to use the existing BOINC file structure and data directory - ie. don't create a new one.  I don't know anything about Windows installs so can't comment on the screens you might see during installation.  You just want to be sure the installer finds your correct account file.