ArthurWalden: Unfortunately, that's not possible. The only way to dismiss an NPC is to try to recruit a 5th NPC, at which point you will be required to choose one NPC to dismiss. However, I am not sure you are correct about experience in this particular game. It is true that in many D&D games experience is divided equally among all party members (such as the gold box games and often in the Infinity Engine games), I am not sure that is the case in the Ravenloft games and Menzobarranzan. Since having fewer than four party members is not really much of an option in these games, it may be that all characters get equal experience regardless of numbers. In any case, the games really are designed to be played with four characters. Are you particularly obsessed with leveling up your characters as high as they can go for its own sake? If so, other RPGs might be a better fit than these games. If you want to focus on plot, environment, and atmosphere, however, these games are great and you definitely can (and probably should) complete the game with a party of four. Again, my opinion is that the games play best with four characters.
Note: do heed my warning about level draining undead (and shadows, which again are bugged) in Strahd's Possession. Stay out of melee with them as there is no way to restore lost levels (or strength points) in this game. This is only an issue in this game. Menzoberranzan has no undead at all and in Stone Prophet they shadows work properly, making them much less of a threat. (In case you're not aware, unlike level drain, shadow strength drain is supposed to be temporary. However, in Strahd's Possession, it is not.)
Thanks, I appreciate your insight.
I have read from multiple sources that the EXP is divided evenly amongst the party, and fewer party members means more EXP to go around. I just wanted to give my created members a small boost before going full bore with a 4 member party. No biggy.
FWIW, I beat Menzo when it was current, but never got around to the Ravenloft games back then. I'm now playing them on original hardware (I'm a retro PC collector). In the past 5 years I've beaten a number of old DOS RPGs on original hardware, which I much prefer to DOSBox or other forms of emulation.