We learn that Geralt knew the three assassins in this game (Letho, Serrit, and Auckes) but they never mention the one he already killed in Vizima. In the flashbacks Geralt only meets three other Witchers not four.. so who was that other assassin?

Probably an oversight on the part of the developers, really.

Letho even had his medallion during the assassination, unlike the one in the first.

I'm not sure, but I was under the impression that the assassin in TW1 was a result of the theft in Kaer Morhen. *shrugs*

As far as we know, the Kaer Morhen theft has nothing to do with Foltest's assassination. Weirdest thing is that Letho explains nothing on this subject. The game seems to completely ignore the first assassination altogether.

Even if the first guy was a result of experiments, he wouldn't be good enough to go toe-to-toe with Geralt (which he did).

I think that he was one of the two other witchers from Letho's school, the two that he said he knows about but not where they are.

C'mon guys, there is no mystery there. This is a plot hole, devs just forgot about the first assassin closer to the end. :) Try to write a book and you'll understand. There is another one but opposite bug with Selkirk's chainmail: you know about it before anybody shared the info with you. Hopefully, they fix both of these bugs in the next patch.

Actually Geralt is kind a inferior combat wise to first assassin , he almost got his ass kicked , but luckily he had pouch of gold. + first assassin looks a lot like Serrit but fights like Auckles.

But plot holes can be filled with careful explanations in the next game, or even an expansion. I hope that he's one of the two that Letho lost track of, because then there's one unknown Witcher wandering around to be the antagonist in Witcher 3.

As a sidenote, I hope that if that happens, they name that Witcher Ted. It's such an inconspicuous name for a final boss character and I would literally laugh out loud every time Geralt asked about him.

*No offense to any Teds around here. I'm sure you're all fearsome :)

Likely a plot hole but donatelli posted this image from the game: http://www.gog.com/upload/forum/2011/05/3360775235faef83fa7d9faa14adfba1a7cab345.jpg
Looks like 3 guys behind Letho.

I saw that in game. Looks like this picture was drawn earlier in production when they still remembered. :) There is another one where Letho stays in front of only two witchers.

Letho mention the other two were on the path....maybe Emhyr var emries got to them 1st....n he was hired w/o LEtho knowing or sumthing n sick the basted at foltest.....pure speculation