If it does stack then it would be 0.8 (20% reduction) X 0.8 X 0.8 X 0.8 X 0.8 = 0.327
So crits would only deal 32.7% or they would be reduced by 67.3%
at 50% reduced crit damage crits deal the same as normal damage (60% with IE) and anything above that makes crit deal less damage then normal attacks
(with damage reduction being caluculated like that it means that you will never get 0 which is equal to 100% damage reduction unless you have something which gives 100% damage reduction... since no matter what numbers you multiply together as long as one of them aren't 0 you will never get 0... you can get close though)