The Zorki 4K was the 1973 replacement of the Zorki 4. The Zorki 4 had a long run starting in 1956 and outlasted all the other competing rangefinder models of the time. The 4K added an upgrade of a thumb film advice vs the original wheel advance. The 4K did not have the longevity of the original 4 only lasting till 1978.
This 4k has the Industar-50, one of the standard lens options (the Jupiter-8 was the other). This is basically a copy of an early Leica screw mount. There are a few thing to be aware of. No strap lugs. If you want to use a strap you have to use the case. The shutter speed indicator only reads correctly after the film is advanced. Film counter is manual reset. The dial around and under the shutter speed releases the film for rewinding. No light meter.
The 4k has a nice solid feel to it. It is not Leica screw mount quality but still a solid built camera that works the way it is suppose to work. It feels good in the hand and takes reasonable images. It is not the best rangefinder from the 1970s but adequate and takes Leica screw mount (L39) lens. So you can shoot Leica glass, Canon RF glass, USSR lens and a load of other options. This Industar-50 lens is not the fastest at f/3.5 but it gives a great old-school look.
So why do you need one? Other than all the great old lens options, there is the price. Good, working example are a fraction of what you would pay for a Leica screw mount or even a decent Canon RF. Take the time to get one that is working (or supper cheap and take a chance). If you want to experience interchangeable lens range finders without breaking the bank, a Zorki 4 is a surprisingly affordable option. The Zorki is not a Leica, but it can still be fun.
Lens: Industar-50 50mm f/3.5
Film: FujiColor 100