There is no restriction of four wives in the Qur'an
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, the verse that says not to marry 2 sisters together says that polygamy is allowed, but marrying 2 sisters at the same time is forbidden.
If polygamy was forbidden, there would be no need for the prohibition of marrying 2 sisters.
In the same way, pork is forbidden in a different verse because it is halal to eat meat. If eating meat was haram, it would not have been stated in a separate verse that eating pork is haram. Because the prohibition of eating meat would automatically prohibit pork.
And since polygamy is lawful, it is forbidden to marry 2 sisters in a separate verse. Because polygamy is lawful and there is no numerical limit. The only limit is whether women want their husbands to be polygamous or not.
By the way, there is no restriction of four wives. A man can marry up to fifty or more women if the women so wish or if they give permission to their husbands.
