If the officer came with every 1st box, the sapper with every 2nd, the ADC every 3rd and the Medic every 4th that would mean everyone gets as many different figures as possible. Good for everyone.
If the bonus figures were distributed randomly this might lead to universal disappointment. Newton's third Law of Cornflakes box giveaways states that your chances of receiving a given figure decrease in inverse proportion to the strength of your desire to have it.
If you want to reward people who placed larger orders by unlocking each figure based on number of boxes bought that would be understandable but no point asking us where the cut of points should be. You know that we all think the number of boxes we bought should qualify us for receiving all 4 figures. Which means you have to make that decision yourselves.
If there was one of these figures that came with every box, I think it should be the officer. These sets could use extra officers.