Others here my have differing opinions, but personally, I would never attempt an upgrade on such distant versions... 4 to 7. A LOT changed between just 6 and 7, nevermind 4. If your /home is on a separate partition (as it should have been), you can attempt to install 7 at /, leaving /home alone (no formatting). However, with such distant versions, even this may cause problems. If it were me, I'd save the user data to CD/DVD and clean install the whole shebang.