[bp/r50450][SEC] installer: Convert shortcut to step 4 into a $_POST instead of a $_GET so that it works (thanks Arild for testing)

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/9.x@50452 b456876b-0849-0410-b77d-98878d47e9d5