Montpellier miss chance to go second
Teenage Nigerian midfielder Lukman Haruna was instrumental in the home side's win with two goals. Defender Sebastien Puygrenier and Uruguay midfielder Diego Perez scored the other two.
Montpellier, back in the top flight after a long spell in the second division, stayed third, 10 points behind leaders Girondins Bordeaux, after a game that was postponed in November because of an outbreak of the H1N1 flu virus.
The visitors were 3-0 down when they had midfielder Younes Belhanda sent off for a second booking. Monaco climbed to seventh in the 20-strong league.