What is the overall (X-fold) dilution of 4 serial 2-fold dilutions?

16-fold dilution.

Explanation:

A serial dilution is any dilution where the concentration decrease by the same quantity in each successive step. So, for a 2-fold dilution, the concentration decrease 1/2, it means that if we have a sample with 10 M of concentration, after a 2-fold dilution it will be 5 M. For the next step it will be 1/2 of 5 = 2.5 M, and successively.

Then, we just multiply the factor for each dilution. After 4 serial dilutions:

1/2 x 1/2 x 1/2 x 1/2 = 1/16

So, it would be a 16-fold dilution in the end.