If Avast 5 is working correctly, and there appear to be no reliability or performance issues, there is probably no need to run the uninstaller now.
I realize words like "probably" do not make the answer you want to hear. Sorry, I just don't know. The only way to find out would be to search through all folders and the registry, and locate any leftover parts of Avast 4. Not an easy task. More difficult would be to determine if any of those leftovers are actually having an adverse effect - or doing anything at all really. (They "probably" are not.)
Which is why we always recommend the uninstaller is run between installs. (And almost always recommend the use of other programs uninstallers, too, in respect of those programs. AVG, for example.)
The reason to run it in safe mode is that certain files can be loaded with the OS starting up in normal mode. Once locked by the OS, these files can not be easily deleted. The use of safe mode prevents a considerable number of files loading at start.
If it was run in normal mode, it will clean up remnants that are not loaded, and remove any leftover reg entries. If the program had previously been successfully uninstalled, it's my belief that that action will be adequate, as there would no longer be any instructions to load (and thereby, lock) any leftovers to run at start; a simple cleanup is all that's indicated.
If you now run the uninstall utility, whether in safe or normal, it will remove any old Avast entries, and your new Avast 5 installation. If you choose to do this, I'd uninstall the current installation first, follow any reboot commands, run the uninstall utility -either in safe or normal, your choice - reboot, then reinstall Avast 5.
Google Chrome is a browser, like IE8, or Firefox, or Opera. You may use it as such; it should appear in the "all programs" list if you click the "start" button. (A lot of users seem to like Chrome; I haven't tried it myself.) If you don't want to keep it, it can be uninstalled via "add/remove programs", either using the Windows control panel, or the same facility (which runs the same commands- that is, it is merely a different way of doing exactly the same thing,) in Ccleaner.