That's the point, at least 4 witnesses are required.
This is not entirely true. Depending on the fiqh a jurist subscribes to, this may or may not be the case. Remember, Zina is not simply just sex, kissing is also considered a part of the definition of Zina as well other acts that have expounded the definition however since Zina is a Hudud sin, it depends on how the jurists interpret it:
"Abu Huraira reported Allah's Apostle as saying: “Allah has decreed for every son of Adam his share of zina, which he will inevitably commit. The zina of the eyes is looking, the zina of the tongue is speaking, one may wish and desire, and the private parts confirm that or deny it.”
—
Sahih al-Bukhari, 8:77:609, Sahih Muslim, 33:6421
Not only that, they have to be men.
Where does it say that specifically four men?
Shaykh ‘Abd al-Rahmaan al-Sa’di said in his Tafseer (1/563):
“Why did they not produce four witnesses?” [al-Noor 24:13], i.e., why did the accusers not bring witnesses to the things they accused them of?
“four witnesses” i.e., of good character. “Since they (the slanderers) have not produced witnesses! Then with Allaah they are the liars” – even if they are certain about that in themselves, they are liars in Allaah’s judgement, because Allaah has forbidden them to speak of that without four witnesses. Hence Allaah says: “Then with Allaah they are the liars”, and He did not say “Then they are the liars”. All of this points to the grave sanctity of the Muslim’s honour, so it is not permissible to make accusations against it without establishing testimony."
The witnesses have to be of upright character does not specifically mention
only male or female.
Not only that, they have to have seen the actual penetration.
It doesn't say that, now you're making up s*****. Quite obviously when one cheats it's (more than likely) in the confines of privacy which is why the accusation of Zina is very serious because the Qur'an specifically warns against falsely accusing someone of a sinful act.