Mind-boggling applications of Sharia

According to this column, rape can only be proved under Sharia law if there are four witnesses.

OK, I can grant that ancient law can be perplexing to us moderns--see the "Torah" for many an example--but I cannot for the life of me envision why anyone at any period of time would require a rape victim to provide four witnesses.  It's not as if (Genesis 19, Genesis 34, Judges 19) the ancients lacked object lessons as to why a potential rapist might hesitate to perform that abominable act in front of four men who might testify against them.

(shouldn't those four men kill the rapist/s, or die trying? )

If indeed the Qu'ran condones this practice, I'm having more than a little bit of trouble seeing how Islam can--along with Sunni veneration of Mohammed's 9 year old wife Aisha, honor killings condoned by Sharia, and so on--become a religion of moderation suitable for respect in the modern world.