For anyone with Z axis issues, try this:
I think this mod solves what in my opinion is a slightly overlooked adjustment possibility in the mk4 design, the distance between the Z screws and rods. If this distance is not the same at the nut and at the base of the motor mounts all kinds of bad things happen! Excessive wear, Z wobble and even binding up the whole carriage leading to skipped Z-stepps etc(happened to me) it's specced to be 10mm but mine was around 9.4 or so... now, it's spot on.
I had some rather big problems at first calibration as explained here: https://forum.prusa3d.com/forum/postid/772566/
I eventually got it to work and I ended filing the screw holes to shift the motors towards the print bed. This made the printer work and initially I found it was really quite good, I did eventually start to notice the there was some Z wobble during the first 30mm millimeters that gradually faded with height so i figured it still wasn't good enough, I knew I was still about 0.3mm off.
So I decided to modify the mounts the make it possible to shift the motors in the X axis and now things seem to be pretty much perfect!
It was fairly easy to change the mounts, doable without disassembling the nextruder but it is a bit fiddly. I recommend test fitting the rods to the new mounts before assembling things. If it's possible to remove the nextruder cable bundle easily you can fit the rods before assembling the mounts which would be alot easier and probably recommended but I was lazy and didn't even explore this possibility ;P
This problem could be solved by adjusting at the trapezoid nut too but I figured the mounts are easier to replace. The drawback here tho is that you have to adjust the tilt and the shift of the Zscrew with the same 4 little hexnuts, the shift distance needs to be set first.. for obvious reasons. But it works well and it wasn't too difficult to dial it in.
I also made a feeler gauge that you can use to check the distance. Print that thing first and check, if your printer is out more than 0.1mm and you're having quality issues, especially within the first 30 or so mm of height, that might just be the problem.
And thank you to Prusa for publishing step files! awesome!! and to think that some companies/individuals thinks or wants this kind of thing to be illegal....
The brackets: https://www.printables.com/model/1533243-mk4s-z-motor-mount-mod/files
The feelergauge: https://www.printables.com/model/1533248-feeler-gauge-97-103mm