I would like to know if I'm consider a sinner for not believing its ok for a husband to have four wives and if I'm considered still a Muslim if I do not believe in having four wives.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

A Muslim is obliged to believe that it is permissible for a man to marry four women for whoever is able to fulfil its condition, which is to be just between them. This is legislated by Allaah, the Most Wise, the Well-Acquainted with all things. Indeed there are many preponderant benefits in marrying four women. So, a Muslim is obliged to submit him/herself to Allaah's Rulings, and not accuse these rulings of not being just, because if one does so, he accuses Allaah with this and condemns the Wisdom of the legislator Who is Allaah, and it is forbidden for a Muslim to do so. So, if your view about marrying more than one wife is as discussed, then you should be careful.

Nonetheless, if the matter is that the woman fears to be harmed by another wife, or fears that her husband may wrong her and the like, then this is not considered as objecting the principle of marrying more than one wife, and this does not affect her faith. For more details on polygyny, please refer to Fataawa 86893, 83644 and 90132.

Finally, it should be noted that a woman should not object to her husband marrying a second wife, as by doing so, she is objecting to one of his rights, and neglecting many religious benefits, like protecting the society from adultery and fornication, multiplying the Muslim's progeny and so forth.

Allaah Knows best.