Witness doesn't mean that those people should only be the witness that saw rape while it was happening, that by the way never happens, but to approve of the evidence as a witness i.e. in this case the DNA report and if four persons (technicians in this case) approve of this proof of the guiltiness of the convict then it also fulfils the decree. Hence the rapist can be given punishment and no other person would even think of committing a rape or otherwise this stalemate between the mullahs and the law would never end.
This really needs a serious thought on the behalf of government officials or else our society will get worse than western societies. May Allah help us all. Ameen