multiple login

These three topics include the solution to avoid multiple login.
http://stackoverflow.com/questions/5323300/how-to-avoid-multiple-login-for-the-same-user
http://stackoverflow.com/questions/2599118/in-asp-net-site-how-to-prevent-multiple-logins-of-same-user-id
http://forums.asp.net/t/1608429.aspx

then you have check this thread solution.

http://forums.asp.net/t/1196897.aspx

 with the help of above solution if user try to attempt sign in 4 different pc then it is working 
OR
this solution is only for single pc and browser only .

hi,
refer to this link it may help you:

http://geekswithblogs.net/Frez/archive/2010/05/17/preventing-a-user-from-having-multiple-concurrent-sessions.aspx