What needs to be understood is that this method of convicting two people of zina is actually pretty impossible. The chances of four pious males all stumbling upon an act of penetration, and then all deciding to report it (instead of hiding the sin, which Islam encourages), is very, very slim. This has been done on purpose because the punishment for zina (rajm) is very severe and is not meant to be carried out in the first place. It is only ordained to scare the masses and prevent the sin of zina. The only logical way a person can be convicted of zina is through confession, and even then the qadhi must look for excuses to not carry out rajm. I suggest both of you continue reading on the PDF link I provided from page 25. It explains everything.