absolutely right, my mistake, there are five accepted "monuments".
paris-tours is similar in stature to amstel, gent-wevelgem, fleche wallone, etc.
incidently paris-tours is one race eddy merckx didn't win
the greatest classics rider was eddy merckx with 19 classics wins, although rik van looy with eleven wins is the only rider to win each one.
the list of accepted classic "monuments" is:-
milan-san remo
tour of flanders
paris-roubaix
liege-bastogne-liege
paris-tours
tour of lombardy