In Spring 1922, Poeymirau and Freydenberg launched attacks into the headwaters of the Moulouya in the western Middle Atlas and managed to defeat Said, the last surviving member of the Berber triumvirate, at  El Ksiba in April 1922.  Said was forced to flee, with much of the Aït Ichkern tribe, to the highest mountains of the Middle Atlas and then into the High Atlas.  Lyautey then secured the submission of several more tribes, constructed new military posts and improved his supply roads; by June 1922, he had brought the entire Moulouya Valley under control and pacified much of the Middle Atlas.  Limited in numbers by rapid post-war demobilisation and commitments to garrisons in Germany, he determined not to march through the difficult terrain of the High Atlas but to wait for the tribes to tire of the guerrilla war and submit.  Said never did so, dying in action against a groupe mobile in March 1924, though his followers continued to cause problems for the French into the next decade.  Pacification of the remaining tribal areas in French Morocco was completed in 1934, though small armed gangs of bandits continued to attack French troops in the mountains until 1936.  Moroccan opposition to French rule continued, a plan for reform and return to indirect rule was published by the nationalist Comité d'Action Marocaine  in 1934, with significant riots and demonstrations occurring in 1934, 1937, 1944 and 1951.  France, having failed to quell the nationalists by deposing the popular Sultan Mohammed V and already fighting a bloody war of independence in Algeria, recognised Moroccan independence in 1956.
Question: How many different years were there significant riots?
Answer: 4