Allowed, but poorly worded.  The seller obviously doesn't know how to accurately reflect their offer, the listing should have a variation for the different quantities on offer, 1 for $20 or 4 for $60, and photos of a single cartridge and the four cartridges attached to the variations, so that the buyer has to select their quanity and the price and photo would then reflect the offer correctly.  Otherwise the way they have it set up is likely to cause them trouble if the buyer doesn't read the description and sees 4 cartridges and expects to pay only $20.