It has been a while since the Ursa Mini 4K arrived on the scene. The original 4K sensor has a nice look of its own. You will have to learn to live within your means related to dynamic range and performance in low light.
Given the passing of time, the focal reducer kits which were being made for the Ursa Mini 4K and Ursa Mini 4.6K may have been discontinued but as the Ursa Mini form has continued into the 12K, they may still be available.
So far as I know, I made the first one for an original "big" Ursa using the optical cell out of a Metabones Speedbooster "Ultra" 0.71x.
One manufacturer/vendor that I know of operated under the product name Lucadapter. It appears he is still operating. -
https://www.lucadapterstore.com/The focal reducer confers some benefits :-
It fits within the throat of the camera behind the lens and does not introduce any added distractions when setting up.
It increases the field of view that the sensor sees for a given lens focal length.
It passes more light to the sensor.
In practical terms, the Ursa 4k camera then behaves as if it is an Ursa 4.6k camera. You will have to use external ND filters and with added IR filters or IRND filters for best performance from your lenses.
The focal reducer has some downsides :-
It adds more glass between the lens and sensor which potentially adds flare.
Some lenses with very precise infinity focus end-stops in the mechanism may not achieve infinity focus. I understand that the manufacturer may have dealt with that issue since it emerged.
Some lenses may no longer be usable due to deep penetration in the camera throat. Any of the lenses approved for the Metabones Speedbooster 0.71 "Ultra" optical cell should be fine.
The image from some wide-angle lenses may go soft in corners.
The lay of the optics in the focal reducer may cause a soft side in image. This was an issue with some Metabones Speedboosters because of the locking system which was a small screw impinging upon the adjustable optical cell on a radial axis. I think the Lucadaptors may use a different retention method.
This was fixable by simply removing the screw or backing it off, adjusting the cell to correct focus. Then place the Speedbooster facing directly downwards for gravity to centralise the optical cell in its adjustment threads.
Then apply a few small dabs of water-cleanable white bathroom sealer across the thread junction and allow it to set before disturbing the Speedbooster. Do NOT use a silicon sealer as this stuff is impossible to shift if it gets on coated optics. Do NOT use locktite. The optical cell is completely round with no means of securing any lens tool to it.
This linked clip is not the best and was edited as 1920x1080. In the 14mm Sigma for Nikon image at the tail end of the clip, you will observe a sharpness falloff in the upper left corner. there will be better demonstrations on youtube if you search for them.
This is most likely too much information but I get carried away at times. Ignore at will.