Nope.
Once you open the file in 6, and save it, it can never be opened in a previous version again.
If you need to recall the rough edits, you can save out an EDL in 6 and open the EDL in 4 or 5 tho.
EDL- edit decision list. It is basically a text file with a table of the edits made in a project. In some cases, an EDL can be imported into another editing platform such as AVID or Premiere or another version of Vegas and the project can be duplicated in that system.