Hi Martin, that STEP file contains triangle mesh data, not CAD surfaces and solids which is what MoI is generally expecting to be in a STEP file.

I'll take a look at why it would load in MoI v3 and not in v4 but I do not generally expect for this type of faceted triangle mesh data to load well into MoI in the first place. I don't recommend trying to work with polygon data in MoI, it's not designed for that.

- Michael