Here is one possible approach.
Add an auxiliary category. Add 4 manual grade items, one for each quiz item and place them in the category. For each item set the min to 0 and max to 1. Set the formula of each item to something like:
=[[respective_quiz] ]-69
Set the category aggregation method to 'Lowest grade'.
Now, the grade of this category will be 0 if at least one of the modules is below 70, and 1 if all are 70 and above. And so you have a flag which you can use either for display purposes or for further grade manipulations.
(You can hide the category and all its items or only the items, depending on how you use the flag)
hth