Allah Taãla has given certain priviledges only to Rasulullah (Sallallaahu Álayhi Wasallam) which are mentioned in Chapter 33 vs 50-51. Among other priviledges one was having more than four wives simultaneously.
Allah Taãla says, 'Oh Nabi (Sallallaahu Álayhi Wasallam) we have made permissible for you your wives.' (Ahzaab 33-50).
The polygamous marriages of Rasulullah (Sallallaahu Álayhi Wasallam) to more than four wives simultaneously should be viewed as obeying the order of Allah Taãla to do so. It was not merely a means of fulfilling his carnal desires as many people allege. The Arab environment was infested with many evils among which was adultery and fornication. Despite living in such an environment, he never indulged in this evil act. His modesty was so popular among his people that when the Prophet (Sallallaahu Álayhi Wasallam) claimed prophethood, his people made every effort to disgrace him by accusing him of being a poet, practising witchcraft, of being insane, etc.