Try a quick reboot of the router then try
http://192.168.1.1 again, sounds like 192.168.1.4 had an open connection to the admin pages and hasn;t dropped it, or you;ve somehow restricted access purely to that IP..
If the later change your PC to a manual address of (192.168.1.4 with a subnet of 255.255.255.0) and then try 192.168.1.1 again