You can install both DS4 and DS6, they won't conflict with each other. But there's been a lot of complaints especially with the viewport speed so I wouldn't jump on installing unless you need to. Haven't tried it myself yet, just going by the discussion over at the Daz forums.  https://www.daz3d.com/forums/discussion/763286/what-actually-is-new-in-daz-studio-6