nuendo 3 and 4 compatibility

Yes, but no backwards, unfortunately. Same stupid backward in-compatibility as with Vegas projects. That's why long live king Adobe.

Save another version of your project and not over the same project.
 
Yes, they open in 4, but not correctly at all. Some plugins, specially Waves, N4 doesn't know their location, so you have to reload them. Also, if you save N3's track settings, when you load them in 4, they miss the PAN setting.
 
To me works perfectly. I only has issues with N3 legacy plugins and some DX plugins, but that is all.