automatically myltiply quantity by 4 when adding to cart.

[tumanob]

I could not find any plugin or default feature to do multiplication of the quantity by a number. 

 

let's say we have price for the item 10USD but we sell this items by boxes and there are 4 items in a box. So, when user add quantity 2 this means that he want 2 boxes with 8 items. So I need that quantity multiplied by 4 each time user click "Add to cart" button. 

 

I have found Russian  website that has this feature http://maxline-tm.ru/item/38#!prettyPhoto

when you add to cart the quantity auto multiplied by 4.

 

If anyone can help me with that or tell me where to look for that changes? 

 

Thank you.