This is how it works..
on tesco groceries website it says £11 but when you add to the basket it takes £5 per box. to get it to £4 per box use code GRTTRR. this code will give you £4 off when you spend a total of atleast £20.
so add 4 boxes at £5 to the basket making a total of £20 and apply the code above to take off £4 leaving you to pay only £16 for 4 boxes. hence £4 each.
i have also added pictures in the comments