Yes, you can load 03 over 08. Sorry that 08 didn't work. Once you applied 03. I would let the 4500 run for a while with out changing or doing any configuration. I would like to see if it runs for a while with out issues if you can do that. Let us know how it goes. I presume that the time you applied the 08 FW and the 6 hours up time isn't the same?
What is ur ISP, Cable or DSL? I have seen my buddies 4500 have issues with up time. Come to find out it was the ISP and his DSL lines causing issues. Just additional info.