Nina,
XPress v6 saves back only one version, to v5 (just like v5 saved back only one version, to v4, and v4 saved back only one version, to v3). To open a v6 doc in v4, you'll need someone with v5 to save it back to v4 -- or you can look at Markzware's Markztools, which is supposed to open v6 in v4. I've heard mixed reviews on it, though.
peace,
Linda ยท 8)