According to people verse 24:4 says that four witnesses are required to prove the scene of Adultery. If they failed to bring four witnesses and failed to prove their charge then 80 lashes for each of them according to the Quran and their evidence will not be accepted ever after this failure.
According to my understanding requirement of four witnesses is not about witnessing the scene of Adultery or Fornication (Zina) but the requirement of four witnesses is only in case of some people launch a charge against honorable women 24:4.
24:4 And those who accuse the independent/honorable females, then they do not bring forth four witnesses, you shall lash them with eighty lashes, and do not accept their testimony ever; and those are the wicked.
24:5 Except those who repent after this and do good, then God is Forgiving, Merciful.
So the four witnesses are required to prove the character of women, this has nothing to do with the Adultery or fornication (Zina) mentioned in verse 24:2-3.
We know that it is very difficult to bring four witnesses to prove the scene of the Adultery.
Also this verse was revealed 1400 years ago and there was no technology like video recording, cameras, Mobile phones, cctv cameras and medical exams like DNA tests etc were available at that time.
Nowhere Allah said that four witnesses are required to witness the scene of Adultery or fornication (Zina) but four witnesses are required only in case of some people launch a charge against honorable women.
According to the Quran the Believers are commanded to guard their eyes in verse 24:30-31, So it is not possible for the believers to watch and witness the act of Adultery or fornication (Zina).