Zeldman advocates tough love for 4.0 browser users. I respect his decision and actually have slowly begun to weed out 4.0 compatible markup in my sites, though I feel I can't afford to be one of the WaSP pioneers at this point. Nevertheless, if you are using a 4.0 browser these days, I would ask that you consider the position of the developers of sites that you habitually visit. If a site suddenly becomes illegible and asks you to upgrade to a more standards-compliant browser, perhaps you should consider upgrading (if you can) in an effort to aid the development of that site.
Reasons not to upgrade even though you have the ability and time could also include: 1) You hate the developer working on the site. 2) You want the site to be less productive. Both valid reasons.
Though I try to keep my public sites compatible with 4.0 browsers, I really, reaallly don't with respect to web apps I create. Neither, it seems, does Scott. Scott is a developer who, without prompting or pay, said some nice things about my mini-portal. I wonder if we have similar jobs? For the most part I create web applications that very, very few people will ever get to see or use but which mean a lot to the users who need them to work. He'll never know but one of his articles played an influential part in my current development methodology. Well, maybe he would know if I could just remember to email him. I should do that.
A very funny snippet from the same nice young man: Scott muses on why birds sing.