The storekeeper is down 4 shillings plus the cost of the chocolate. *

The shopkeeper gains 10 shillings from the fake note that he exchanges for the real one. He then gives 4 of those shillings away in change to the customer, so he is at 6 shillings profit. The shopkeeper then has to give those 6 plus 4 more shillings to the tobaccoist. The shopkeeper has effectively lost 4 shillings, and whatever the cost of the chocolate was.
* (It is not likely that the shopkeeper originally paid 6 shillings for the chocolate, because this would mean a loss in profit. Whatever he paid for the chocolate in the first place is what he has lost, plus the 4 shillings in change.)