Not much has yet been written about the challenge of finding compatible high-performance lenses for use on 4K systems. Without a compatible lens, the lens will become the limiting factor in the system and waste the investment by delivering sub-4K images, effectively reducing the overall system image resolution performance.
There are a few popular 4K sensors from Sony, the IMX172 and IMX226. Because the sensor sizes are not much bigger than current sensor sizes (5-megapixel sensors for example), yet the pixel count is significantly higher, that means the pixel sizes are smaller.
Without a compatible lens, the lens will become the limiting factor in the system and waste the investment by delivering sub-4K images, effectively reducing the overall system image resolution performance.
The availability of lenses which can resolve these small pixels is limited. There are some fixed focal length machine vision lenses offered by some of the camera manufacturers. Availability of varifocal lenses which give the installer more control over fine tuning of the field of view (and also fewer lens types to order) is very limited but should increase in the next few years.
The smaller 4K image sensor (1/2.3 inch) will be used for the lower cost 4K cameras. The small pixel size will cause reduced image quality in low-light situations because the pixels cannot physically collect as much light as those of the larger (1/1.7 inch) 4K image sensor. And the smaller 1.55-micron sized pixels of this sensor requires the lens be able to distinguish 300 line pairs per millimeter with good contrast, a challenging task requiring high precision optics. On the other hand, the larger 1/1.7-inch sensor with a 1.85-micron sized pixel requires the lens be able to distinguish only 270 line pairs per millimeter with good contrast — a somewhat easier task, but requires larger lenses, making both sensor and lens will be more expensive.
The 4K cameras and lenses are still made in low volume so that also makes them more expensive. And higher resolutions lenses made to support the smaller pixels increases the complexity and cost of the lenses. But without these high performance lenses, the investment is wasted and the promise of 4K is not achieved.