just turning the numbers around (and i might complerelty wrong)
71% chance for one of them to crit means that there is a 29% chance for NONE at all to crit.
Meaning that the 71% actually covers all the times when 1 will crit, 2 will crit, 3 will crit and the (rare but devastating) occasion where all 4 will crit.

In other words, collectively they have a 71% chance that AT LEAST one of them will crit.

I know, probably not the most useful post. nvm