I'm with you - couldn't find the fourth either. I googled it and found the same 3 on wikipedia. The only mention of a "fourth" one was the Dispatcher - but they say it is part of the scheduling, not an actual scheduler.
Well, in my case, I decided to include the dispatcher anyway.
As mentioned, its a part of the scheduling function, but since the CPU scheduler hands the next thread to the dispatcher to be loaded on the CPU, it can possibly be thought of as a scheduler with only two threads to manage (the one currently on the CPU, and the one to be loaded). Thus my reason for including it in the question was:
a) if the question had a typo and asked for 4 types of schedulers instead of 3, then including the dispatcher in the answer doesn't lose any marks (assuming that the onther 3 are correctly named and defined).
b) if the question was correct and was correctly asking for 4 schedulers, then the only possibility (as far as we can determine) is that the fourth must be the dispatcher. Thus by including it, you don't lose any marks.
Ultimately, I just included it, so either way I'm covered for the question.