A UHD stream takes way more bandwidth than an HD stream, and multiple streams take more than one stream. If you just look at it as "How hard are you going to hammer our servers?" the current tiers make a lot of sense, they are just hard to explain to the average customer.
OFC, four UHD streams vs a single SD stream is a massive jump, but I doubt that extreme is common in a single household. Now, if people share passwords, lots of 4K streams starts looking likely.