It looks like you've used some backticks in there when they should be single quotes. Although I haven't tried it at all, replace() is a string function while userid is an int field, so you might want this instead:
Code:
update moderatorlog set userid = 4 WHERE userid = 90
In fact, if the replace() query works at all, it would replace any 90 with 4, so if you had a moderator 190 or above 900 for instance, it would cause problems..