Not sure I'm understanding the question, are you wanting to replace any values > pi/2 with pi/2? (and any values < -pi/2 with -pi/2?)
if you're just looking to plot the function from 0 to pi, you can use the same method that I suggested on your previous question:
f = @(x) atan((x*.8441/4)/(.8441^2-x^2))
fplot(f,[0,pi])
EDIT
I think I see what you're wanting, although I couldn't open your link at work. The unwrap function is very interesting; hadn't heard of it until now, but I couldn't get it to work for this application for some reason.
There are many ways to do this, so I'll go ahead and show you one.
x = 0:0.01:2;
y = atan((x*.8441/4)./(.8441^2-x.^2));
y(y<0) = y(y<0) + pi;
plot(x,y);
note that I had to add some extra '.'s in your equation to make it work.
