I think it’s a big difference,
if you have a good working knowledge of amplifiers, cabs, and how impedance affects their performance.
In this case, the OP told the seller that he wanted to order an 8 ohm cab, so he would have the option of adding a second cab later. From his description, it sounds like he was talked into a 4 ohm cab, with the understanding that he could still add another one, and that it would work properly with his amp. Unless he was told otherwise by the seller, it would be natural for the buyer to assume that he could add the second cab without a loss of performance.
To expect a lay person to understand the difference between a cab setup that will work with a 4 ohm amp and a cab setup that is
rated at 4 ohms, is a large assumption on the part of a vendor, especially if he is dealing directly with the customer. Ampeg can’t control what combination of their equipment people buy off the shelf at Guitar Center. However, if something is being custom built, or sold directly, I think the seller has a much greater responsibility, in terms of meeting the customer’s needs.
I don’t have a dog in this fight, but it has been interesting to watch...