From the, "Did you ever wonder?" files

The 5-5-5-5 fire code - four sets of five rings that tolled
Thursday in memory of firefighters lost in the World Trade Center
attack - has its roots in the 19th century, when telegraph lines
were linked to firehouses.
Jack Lerch, who works in the New York Fire Department's training
academy library, said the signals were a variation of Morse code
used by firefighters.
The telegraph lines were linked to bells in each firehouse that
would clang for various messages. For example, a 1-2-3 series of
signals - one ring, then two rings, then three - indicated a
company was out of service.
Lerch said the four sets of five rings were originally used to
message firehouses that they should fly the American flag at
half-staff for Memorial Day or the death of a president, for
example.
Over the years, he said, the "four fives" came to symbolize
the death of a firefighter. It has no numerical significance, but
dispatchers still will alert firefighters by radio that a 5-5-5-5
signal has been sent.