I'm willing to bet that probably around 99% people using ns4 that went to some website they wanted to use, such as their bank or something, saw something that said 'please upgrade', with a quick painless link to do so, would upgrade.
For some people (my mother?) there is nothing either quick or painless about downloading and installing a new browser. They have a system that works and they do not want to break it. I don't see too many people still running Netscape 4.X but I'd be willing to bet that a fair proportion of those that are still using it are simply too scared or rather too cautious to upgrade.