Hey..if u have seen Harry potter 7 (1st part) answer me this...y did RON say "only three to go" (contd...)?

(contd...) after destroying the locket(horcrux) when there are atleast four more horcruxes(excluding Harry) that needs to be destroyed..was that a mistake made in movie making? only 3 horcruxes were destroyed by the end of that movie? Am I right?