Here is another fatwa on this issue, from the IslamToday.com committee:
Question: Is it really true that in Islam, four witnesses are required to prove a case of rape?
Answer: There is some confusion here. In Islamic criminal law, there are two types of punishment: the prescribed punishment (hadd) and the discretionary punishment (ta`zîr).
Prescribed punishments are those that are imposed by the sacred texts for certain crimes in the presence of certain evidence. When guilt is established by that evidence, it becomes obligatory upon the state to carry out the punishment as a religious duty. The judge does not have the jurisdiction to modify or waive this punishment.
The prescribed punishment for fornication is 100 lashes with a whip and for adultery, it is stoning to death. The evidence required for this punishment to be carried out is the testimony of four eyewitnesses of good character or a personal confession willingly given.
In Islamic Law, a ruler or judge may impose a discretionary punishment (ta`zîr) where no prescribed punishment is given in Islamic Law for a certain crime. Likewise, the ruler may decide to give a stricter punishment than the prescribed punishment.
It could be by imprisonment, whipping and in some cases killing. The Muslim scholars in Saudi Arabia, for instance, have decreed the death penalty for drug smugglers who bring large quantities of drugs into the Islamic society. No punishment for drug smuggling is mentioned in the Qur’ân and Sunnah.
Four witnesses are needed to establish unlawful sexual penetration – whether consensual or otherwise – for the Islamic prescribed punishment of flogging or stoning to death to be carried out.
In the absence of this evidence, the prescribed punishment (hadd) for fornication or adultery cannot be carried out.
However, the judge may use other forms of evidence to establish guilt and impose a discretionary punishment that he deems suitable for the crime.