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Yes, the S4 is a worthy upgrade to your S2. Not only is it running a newer version of Android and blessed with better display. It's also faster (of course) and blessed with a superior camera and compatibility with the Galaxy Gear (should you ever want to try that). However, I might add, unless you're married to Samsung you should consider the other top contenders for 2013 as well:
The Moto X (almost endlessly customizable with great features and performance)
The HTC One (now available it lots of colors for it great build quality and delicious display),
The LG G2 (my device of choice and great phone with a great display and camera as well).
The Nexus 5 (great price on a super phone...no way the S4 is better than this puppy).
That written, if you should decide on the S4 know that you'll have a great phone with lots of support from developers for custom ROMs should you decide to try something even fresher.