The Z4 is a pretty decent camera if you get a good one, but is by no stretch of the imagination as well built as a Leica, Canon or Contax. That said, you can get excellent results if it is sorted. When you are doing your film tests, I would suggest you verify the close focusing of your J8. These are really sharp lenses, but quality control was quite poor in the adjustment of the focusing mounts, and they often need to be adjusted.
First, check the accuracy of your rangefinder by sighting something as far away as possible to see if the images come together when the focus is set to infinity. The moon is the very best target for that, but if the weather is not cooperating, anything more that 100m away will do as well. If it is not exactly on, remove the screw on the front just beside the viewfinder window: under it you will find another screw that can be adjusted with a small screwdriver- about 1mm is best- to get it exactly correct.
With the RF accurate at infinity, take pictures up close, about 1.2-1.5m, with the lens wide open. It is best to use a subject that has some depth to enable you to determine if the lens is focusing too close or too far. If the lens focuses where the rangefinder says it is, you are in great shape, if not, you will probably need to adjust the shimming in the lens. this is notvery difficult, and there are many posts on RFF showing how to do it.
Best of luck!
Cheers,
Dez