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So as fans of the series will rightly remember a witcher tries to kill foltest at the end of TW1.

Geralt kills him.

Near the end of the witcher 2 when Letho is talking to you about his plans and also your memory is returning only Letho and the two other witchers who you kill in chapter 2 are ever mentioned in geralts memorys.

No one ever mentions the 4th witcher that got killed in the end of TW1. They say letho's group from the school of the viper nly cnsisted f three witchers when they get caught in nilfgard.
At the end I seem to remember Letho mentioning two other Witchers from the Viper school or whatever. Maybe it was one of them?

That would set up one unknown Witcher as an antagonist for a sequel, but I doubt that's what they were thinking. More likely it was just an oversight.
There are a couple of possibilities. Can't fully remember, but it's possible it was another witcher with Letho who went on ahead. Or it could be a witcher created by the salamandra who met up with Letho and tried to kill Foltest.

Tbh, I'm not entirely sure, it does seem like an oversight, but one that has potential theories to explain it.
I guess Letho was busy in Aeridan to assassinate Demavend, but he heard about the troubles in Vizima (the story of TW1) so he sent the witcher to take the advantage of the sitiuation in Vizimia. But that failed because Geralt was in the right place at the right time.