1,2,3 or 4

Had known

KNEW/HAD KNOWN если он уже умер; HAVE KNOWN/KNOW если он еще жив.

Они сами не знают точно))
https://english.stackexchange.com/questions/254467/reported-speech-why-is-the-past-perfect-the-right-answer-here

Very interesting, but I don’t agree that (a) is incorrect. We say that all the time.

Uly, so you're saying that this version is correct?
"He said that they know each other for many years"
Aren't supposed to use KNEW here because it starts with "HE SAID"?

Not necessarily. When you want to stress the present validity of something, you can use the present: “He told me they know each other for several years.” We use this form when we want to say two things at once: (1) yes, we know each other; (2) not only do we know each other, we’ve known each other for a long time.

https://www.goodreads.com/en/book/show/23997295-winning-bracket (Arianna’s comment)

“Winning Bracket” is sweet and fluffy frenemies to lovers story that for sure it will put a big smile on your face. The story is about two college guys, Oliver ‘Ollie’ and Edwin, both RA in the same dorm. They know each other for several years and they want each other since the moment they met. But they also drive each other crazy. Ollie is the guy who is loved by everyone in the dorm while Edwin is shy and not as social as Ollie.

In the snippet above there's no past and present within the same sentence. Not sure if the snipped is right for our case.
In any case, your explanation shows that we can use the present if there is an additional phase/fragment that is pertinent to the sentence.
Am I right?

The problem is that if you say “He said that they KNEW each other for several years,” it sounds like one of them died.

Вот это вопрос на засыпку, оказывается)

Reported speech is speech which tells you what someone said, but does not use the person's actual words: for example, 'They said you didn't like it', 'I asked her what her plans were',

…died or has since disappeared

Cool, thank you both again.

А в этом примере всё нормально? -- 'They said you didn't like it'

didn’t/don’t да