It was only possible to play as Kitana on the PC via a third-party trainer or on the N64 via Gameshark codes. On the PS you could enable Shinnok to fight using Kitana's moves via Gameshark codes, but not actually play as Kitana.
To explain, the reason you could only play as Kitana in the N64 version and not in the PS version was that her character model was only used in the N64 version. Due to the fact that N64 cartridges didn't have enough room to store FMVs, the actual character models from the game were used to act out the endings in MK4, and since Liu Kang's ending had Kitana in it, they had to create a 3D model for her. Of course, since all endings appeared as FMVs in the PS version, her model was nowhere to be found in that version of the game's code.
That's it. There was no "legitimate" way to play as her (i.e. the way you pick Goro or Noob Saibot). If you've heard anything else on the subject, it's BS.
Really, though, if you want to play as Kitana in MK4 that badly, just pick up a Dreamcast and a copy of MK Gold off of Ebay. The half-assed Kitana you could sort of access in the home versions of MK4 was not the Kitana we all know and love.