Hi,
Yes and no.
You can't directly do such discount.
However, you could create product C with the same name as product B and with a cost of £4 in an unpublished category and add it as an option of product A so that on the product details page of product A, the customer would be able to add product C as an option of product A when adding product A to the cart.
And if you have the "group options with product" setting activated, the customer won't be able to remove product A from the cart without removing product C too.
And if you want the stock of product B to be updated when product C is bought together with product A, you could add product B in the "bundled products" setting of product C.