I'm guessing this is intended but not expressly stated, when rolling penalty dice (say for casting a spell above your IQ) should fumbles be more likely?
As stated spell crits and fumbles only happen on special rolls of 3, 4, 17, and 18. A 3 is impossible to crit on 4D because the lowest you can roll is a 4. On the other hand, rolls in excess of 16 are more likely with 4d and almost certain on 5D. Does this mean a spellcasting roll of 19-24 doesn't fumble because it's not exactly 17 or 18? I'm guessing this isn't intended.
I'd go with a roll of 18+ is the worst possible result and a roll of 24 is even worse, maybe the Wizard dies in a destructive blast equivalent to a fireball.