Robert Spencer provides a very nice explanation of the origins of the absurd four witness requirement for cases of rape. The outcome of this requirement is, as you will see below, that it is virtually impossible to charge a man with rape under the sharia:
Pakistan’s President Muhammad Zia-ul-Haq (1977-1988) implemented the Hudood Ordinances in 1979. Hudood crimes are those considered most serious in Islamic law. These laws returned the crime of rape to the realm of Islamic law, making a rape victim liable to being prosecuted for adultery if she could not produce four male Muslim witnesses who would testify that they had actually witnessed the rape.
This law comes from the Qur’an. Accusations of adultery against Muhammad’s favorite wife, Aisha, and Muhammad’s desire to exonerate her brought about the requirement that four male Muslim witnesses must be produced in order to establish a crime of adultery or other sexual indiscretions: “Why did they not produce four witnesses? Since they produce not witnesses, they verily are liars in the sight of Allah” (Qur’an 24:13; see also 24:4 and Bukhari, vol. 3, book 52, no. 2661).
Aisha’s own word counted for nothing to establish the falsity of the accusations against her -- so to this day Islamic law restricts the validity of a woman’s testimony, particularly in cases involving sexual immorality. Says the Qur’an: “Call in two male witnesses from among you, but if two men cannot be found, then one man and two women whom you judge fit to act as witnesses; so that if either of them commit an error, the other will remember” (2:282). And Islamic legal theorists have restricted women’s testimony even farther, limiting it to, in the words of one Muslim legal manual, “cases involving property, or transactions dealing with property, such as sales” (‘Umdat al-Salik, o24.8). Otherwise only men can testify.
Consequently, it is even today virtually impossible to prove rape in lands that follow these Sharia provisions. As long as men deny the charge and there are no witnesses, they get off scot-free, because the victim’s account is inadmissible. Even worse, if a woman cannot produce four male witnesses, she may end up incriminating herself simply by making the charge: she has by charging a man with rape made an admission of adultery. That accounts for the grim fact that, according to the Muslim feminist group Sisters in Islam, as many as seventy-five percent of the women in prison in Pakistan are, in fact, behind bars for the crime of being a victim of rape.