That's a secret (cheeky)
UPDATE : Customer services refunded the extra £4.22 as mne came out at £24.22. so if the deals coming out above £20 just order and contact them. They responded really quick via facebook messenger and refund processed already.
I ended up paying £24 discount even said "4 for £20" then still cme up as £24, contacted CS they've asked for my email and order number so looking hopeful they will refund the difference!