Well, firstly their intended formats are wildly different...
With a 4K camera you are typically (although not always) shooting with at least a 35mm-sized sensor. The 2/3" camera is using a much smaller sensor that is... surprise... 2/3" of an inch measured diagonally. A 2/3" sensor is roughly (not exactly, but in the ballpark) the size of a 16mm film frame. So compare that to a 35mm film frame and you'll see the huge difference.
So why does that matter?
Well, a lens has to "cover" the format that you are shooting. A lens doesn't really make a rectangular picture... it really projects a circular image. And that projected circle, at the film plane, must be at least as big as the sensor... 35mm, 2/3", whatever. Take a lens that is made for the 35mm format and it projects a big enough image circle to cover that sensor. Which also means that it will also work fine (assuming it is the right mount) on a 2/3" camera, 16mm camera, or anything smaller. Now conversely, the image projected by a lens made for the 2/3" format will likely only project an image a little bigger than it needs to be for that small frame. If you were to put it on a bigger-sensor camera, you'll see the corners clipped off, or in radical instances even a black frame with a circular image inside it. Like looking through the peep hole in a door. This is why most all 35mm lenses will work on 16mm cameras, but 16mm lenses will (usually) not work on 35mm cameras.
On top of that, lenses made for higher resolution imaging typically have higher resolution (i.e., are simply much sharper) than those made for smaller-resolution sensors. Take a lens that looks perfectly fine on an old standard definition camera and put it on a 4K camera (or even a 1080 camera) and you might find it noticeably soft. It was only made to resolve as high as it needed to and be as sharp as it needed to be (which also means cheaper), which was for standard-def footage. A $100,000 lens will likely never look any better than a $1,000 lens on that camera. The reverse is not true though for a 4K camera.
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