First this is in the wrong sub-forum, which is for the server version of avast only my curiosity got the better of me got me to read it.
First there are many things that make avast 5 better than 4.8, more functions, better detection rate, lighter on resources, improved User Interface (UI) which is much more user friendly, etc. etc. It really is no contest, if your OS is supported (not win9.x, winME), then you should update to avast 5.
Avast 5 has been out for 8 months, not to mention support for avast 4.8 is likely to cease at the end of this year; hence the update pop-us to avoid everyone trying to update at the very last minute, overloading servers.
Personally I trust youtube as far as I can throw it (which is you can't) with reviews, etc. by so called armchair malware experts, where you haven't a clue how they have tested or what their settings are. You only need look independent testing sites like av-comparatives, etc to see just how well avast compares.
See
http://www.avast.com/pr-avast-free-antivirus-is-a-suite-performerThis relates to this test the Symantec sponsored test at Denis Publications labs,
http://www.dennistechnologylabs.com/reports/security/anti-malware/symantec/, in which avast free did better than many paid suites and 4th overall in the test.