This is an unfortunate misunderstanding of what Muslim law actually calls for. The four male witnesses are required, according to the Koran, in order to convict a woman of adultery. Mohammed had a convenient revelation to this effect when his favorite wife got herself into a compromising position with another man. This requirement, if actually followed, which it often is not, would obviously make it extremely difficult to convict a woman of adultery.
OTOH, I don't know of any requirement in the Koran that requires four male witnesses to the act of rape. And I believe the penalty for rape under Muslim law is death.
Whether these rules are applied or enforced is another matter entirely. And, of course, they probably have come up with some amazing theories about what actually constitutes rape.
But the main problem is not how rape and adultery are legislated in the Koran. It's how these rules are interpreted and applied.