This is a variation on the theme of the old song "10 Green Bottles" - in the song the bottles have been changed to lightbulbs and this represents the line in the song:-
"5 used lightbulbs sitting on the wall,
5 used lightbulbs sitting on the wall,
and if one used lightbulb should accidently fall,
there'd be four used lightbulbs sitting on the wall."
:-D