Salam,
Some interesting comments on these verses, from Muhammad Asad. Quite a lot to read, but it is a worthy analysis, I think.
Some of the commentators attribute to the term fahishah (here rendered as "immoral conduct") the meaning of "adultery" or "fornication" and are, consequently, of the opinion that this verse has been "abrogated" by 242, which lays down the punishment of one hundred stripes for each of the guilty parties. This unwarranted assumption must, however, be rejected.
Quite apart from the impossibility of admitting that any passage of the Qur?an could have been "abrogated" by another of its passages (see note on surah 2 verse 106), the expression fahishah does not, by itself, connote illicit sexual intercourse it signifies anything that is grossly immodest, unseemly, lewd, indecent or abominable in word or in deed (cf. Lane VI, 2344 f.), and is by no means restricted to sexual transgressions.
Read in this context, and in conjunction with 242, this expression obviously denotes here immoral conduct not necessarily amounting to what is termed zina (i.e., "adultery" or "fornication"), and therefore redeemable by sincere repentance (in contrast to a proven act of zina, which is punishable by flogging).
It is noteworthy that in all cases of alleged sexual transgressions or misbehaviour the Qur?an stipulates the direct evidence of four witnesses (instead of the two required in all other judicial cases) as a sine qua non of conviction. For the reasons underlying this injunction, as well as for its judicial implications, see note on 244.
Note on 244 below
By obvious implication, this injunction applies also to cases where a woman accuses a man of illicit sexual intercourse, and is subsequently unable to prove her accusation legally. The severity of the punishment to be meted out in such cases, as well as the requirement of four witnesses - instead of the two that Islamic Law regards as sufficient in all other criminal and civil suits - is based on the imperative necessity of preventing slander and off-hand accusations.
As laid down in several authentic sayings of the Prophet, the evidence of the four witnesses must be direct, and not merely circumstantial in other words, it is not sufficient for them to have witnessed a situation which made it evident that sexual intercourse was taking or had taken place they must have witnessed the sexual act as such, and must be able to prove this to the entire satisfaction of the judicial authority (Razi, summing up the views of the greatest exponents of Islamic Law).
Since such a complete evidence is extremely difficult, if not impossible, to obtain, it is obvious that the purpose of the above Quranic injunction is to preclude, in practice, all third-party accusations relating to illicit sexual intercourse - for, "man has been created weak?? (42 - and to make a proof of adultery dependent on a voluntary, faith-inspired confession of the guilty parties themselves.