The law being discussed makes it mandatory for people arrested. (suspects)
It is the only thing I addressed. The fourth amendment concerns unlawful searchs and seizures.
Well, if the DNA sample is taken from an arrested person because a law permits the government to do it, then by definition that's due process of law. The Fourth Amendment issue is separate -- a law that permits an unreasonable search satisfies due process but nevertheless still violates the Fourth Amendment -- but here as well Congress is entitled to determine whether such a search is reasonable in the cases of persons who are arrested. If the arrestee thinks that the search is unreasonable despite the law, he can ask a court to declare the law unconstitutional, either generally or as applied to him in the particular case. My guess is that he would lose, and deservedly so.