Should I uninstall Avast 4 before installing Avast 5?
If so, is there a special uninstall tool I should use?
I usually un-instal V4.
Download the install first:
http://www.avast.com/free-antivirus-download
avast! 5.x: Uninstallation methods:
https://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=504
You don’t have to but i prefer clean installs as it lesson’s the chances of something going wrong.
There is a removal tool highlighted just above your post, (avast 5 removal tool) it will also delet avast 4.
Download the new avast 5.0.677 and the removal tool and save to download’s, then go to add remove programs and delet avast 4 and reboot into safe mode then you can run the removal tool, then reboot again. Now you should run a cleaning tool like ccleaner just to tidy up and your all clear to install avast 5.0.677 
You need to have window’s 2000 or above to run the new avast and make sure your system is fully patched and updated.