Query for Time at which user limit exceeded

Query for Time at which user limit exceeded

I have a list of time user signs up . I have simultaneous system login
limit lets say 4 users . As you can see in table below at 1/1/2013 8:03 AM
that limit was reached . 1st user signed out at 8:01 . How can it be done
in MYSQL query ?
UserID | LoginTime | LogoutTime
1345 | 1/1/2013 8:00 AM | 1/1/2013 8:01 AM
2324 | 1/1/2013 8:01 AM |
3454 | 1/1/2013 8:02 AM |
4564 | 1/1/2013 8:03 AM |
5743 | 1/1/2013 8:04 AM |
6743 | 1/1/2013 8:05 AM |