It's not just the TV...
It's not just the TV itself, but the infrastructure supporting it. 4K will take a lot of bandwidth, and many cable companies are still relying on an outdated infrastructure. In your typical HFC plant, you've got a finite amount of space between 50 MHz (anything below that is used for upstream) and 1 GHz (anything above that can't be reliably carried by the equipment and cabling). Each 6 MHz slot can be used for a single analog channel (which is why analog cable is vanishing), six or seven digital SD channels, two digital full-HD channels, or about 38 Mbps of downstream Internet bandwidth (shared between everyone on the node).
If you're a scumbag like Comcast, you stuff three HD channels into a slot instead of two, at the expense of having to compress it to the point where people start noticing artifacts. How much bandwidth is going to be required for a 4K channel? For it to look decent you probably have to consume an entire 6 Mhz slot.
Verizon and other FttH providers will be able to handle it, but the vast majority of providers who are still on HFC will have a huge investment in front of them if they want to keep up. This could stall the rollout of 4K.