var browser, version;
browser = navigator.appName;
version = navigator.appVersion;
var ie4 = (browser.indexOf("Explorer") >= 0) 
&& (version.indexOf(4.0) >=0 );
if (ie4 == true) {
// do something, for example;
// location.href="indexie4.html";
} else {
// do something else, for example;
// location.href="indexnn4.html";
}