I have an 82 4M and a 78 4, both of which suffer light leaks. Depspite me stripping it down and applying the fixes suggested on the Kiev Survivors website, the 4M stubbornly continues to fog the bottom left of each image (light leaking from the top right of the frame, looking at the back of the camera) and suffer overlapping frames when the advance mechanism somehow slips.
I am not sure about the much smoother 4, but have given up on these late and not terribly well made Kievs in favour of a 1953 2 that a) worked perfectly straight out the box and b) is 95% as well made as the pair of 1930s Contax IIs that also work perfectly and show not the faintest sign of any light leaks or film advance problem.
Lots of people have said on these forums that the 1940s, 50s and 60s Kievs are better made and my own experience tends to confirm this. However, that's not to say that a good camera tech, rather than a DIY merchant like myself, wouldn't be able to fix your 4AM!