Hi Will, thanks for sending the test file. I can repeat the difference between v3 and v4 with your file. It appears to be caused by differences in the joining process when importing an IGES file.

For IGES imports, V3 would only attempt to join edges to other edges where the edges matched each other at their endpoints.

In V4 this has been modified so that during the joining process it will also try to split edges at "T" junctures so those will join as well. This is however going poorly with your file here, it's making things like little tiny edge fragments which then makes for an ambiguous face adjacency traversal later on which is used to gather faces on the same side of a non-solid joined surface for orienting normals.

I will send you a v4 test build that restores the same behavior as V3 for IGES joining. If that works better I will revert this v4 change back to v3 behavior.

- Michael