there would be major changes between Service Releases, normally just bug fixes etc.so the upgrade will be straight forward. See the attached release notes for SR 5.
SR updates are normally very quick and painless. The main thing is to check that there are no bugs in the release that could affect your QlikView applications.
Even if you're not updating to the latest release, it would be wise to read through all the later release notes to see what has been fixed (or what bugs there may be in your release).