I can understand why you might feel betrayed, but in reality, this has nothing to do with betrayal.
You dumped her while you still liked her after 4 years because it wasn't working. She is now dating someone else, whether she knew him before you broke up or not, it really doesn't matter now. You were together for 4 years, so I doubt she was using you either, unless you have something to back that up.
Getting hurt sucks, but you'll get over it.
"All is fair in love and war." - Francis Edward Smedley