Setting minimum and maximum purchase quantities is really useful for controlling how many of a given item can be purchased at once, and in with increments. To access this feature, on the product edit screen, uncheck the box labelled "Can buy any quantity."

When this is unchecked you will see a new area appear below. To add your first increment, click + Add Increment

Enter the minimum and maximum of the quantities you want with the increments you want to use. For example if you want a product to have a minimum buy quantity of 4 and a maximum buy quantity of 1000. You would enter the following: 

To make sure no-one can purchase under the minimum amount of 4 you must then cover the range of 0 to 4 with the increment matching your intended minimum value.

If you want the user to buy in a set amount of units then you change the increment. For example any quantity divisible by 2 and between 4 and 100 would look like this:

You can have multiple increments for higher values. This example shows: below 4 you cannot buy; between 4 and 1000 you can buy in sets of 2, and above 1000 you can buy in sets of 10 only with no maximum limit.