The density of the original culture was 4 × 10⁶ CFU/mL.
I presume that you mean 74 CFU (colony forming units).
Note: The answer can have only 1 significant figure, because that is all you gave for the size of your aliquot. If you need more precision, you will have to recalculate.
The formula I find in my searches is
It gives the same numerical answer, but it requires memorization.