Emmanuel,

Although I'm not a Windows user, I installed and ran the beta 4.1. Using Wine, it ran without any problem, although it didn't recognize the plugins I have installed.
Moreover, when switching back to v4.0, SH3D also doesn't see the installed plugins anymore.

This is a bit strange, because the plugins are still in their directory .eteks/sweethome3d/plugins.

UPDATE
Restarting my computer solved the problem: the plugins are back.

Hans