This should do it:
var newClass = window.location.href;
newClass = newClass.substring(newClass.lastIndexOf('/')+1, 5);
$('body').addClass(newClass);
The whole "five characters" thing is a little worrisome; that kind of arbitrary cutoff is usually a red flag. I'd recommend catching everything until an _ or .:
newClass = newClass.match(//[^/]+(_|.)[^/]+$/);
That pattern should yield the following:
../about_us.html : about../something.html : something- .
./has_two_underscores.html : has
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