I did a quick calculation, and there should indeed be a substantial shift in the focal plane using an infinity corrected 4x lens with the standard optics module designed for a tube length of 160mm.
If you print the infinity corrected optics module, it would be a good idea to check the tube lens position as well, as in this thread, posts 12-14: Focusing and Chromatic Aberration Issues - #12 by j.stirling . There is a possible issue with the module design that might leave some remaining focus issue with your low magnification.
When you have the optics working, you might find it useful to try different motor gearing to speed up the motion. @MilliRowland tested a little a while ago and there did not seem to be an issue with moving from 2:1 to 1:1 or even 0.8:1 on the gearing to make it twice as quick (Faster motion, different gearing). There is a merge request to implement this option at https://gitlab.com/openflexure/openflexure-microscope/-/merge_requests/352. The gears linked from the forum post are based on the old push-fit version. Gears based on the new version, locked with screws, are untested, but these are the STLs if you wanted to try:
large_gears_ratio_0.80.stl (803.0 KB)
small_gears_ratio_0.80.stl (1.1 MB)
large_gears_ratio_1.00.stl (893.6 KB)
small_gears_ratio_1.00.stl (1.0 MB)
At the different ratios the ‘small’ gears are large, but the naming remains that the small_gears_????.stl go on the motor.