Well, the notion of it working "well" is debated in this forum time and again - haven't had any problems myself in the VERY LIMITED amount of time I've done it, but you'll read some horror stories from some others.

Yes, you can load native Poser6 characters with clothing - seems the consistent complaint is dynamic properties like certain hair characteristics can sometimes be problematic.

You can also save them as OBJ (instead of native PZ3) files and import easily - you just can;t make changes in Poser and have them automatically updated the next time you open Vue.

Dunno specifically if it'll load everything you've got from v4 - but there's a folder that's labelled Poser4 in my copy of DAStudio - which means it's in Poser6 as well.