Tim Bray's company Antartica is dropping support for 4th generation browsers, specifically IE 4 and NS 4. A year ago, I was trying to do exactly that for 3D-Secure. At the time, IE 4 had less than 4% marketshare and NS 4 had less than 2%. I figured that, by the time 3D-Secure becomes common place, these old cranky browsers would be gone. Today, each browsers have about 1% marketshare. While some might argue that 2% is still significant, problems caused by supporting them was way out of proportion to any benefits. Good riddence.
Visa and MasterCard, its time to drop IE4 and NS4 support. If you really want to better serve your customers, add better Mac browser support, including Safari.