I have both, and I love both of them. However, I can see that the 4 will have a definite advantage in the future seeing as the 3 is basically as good as it's going to get software-wise. Not only does the 4 have the App Store, but the app creators have more control over the apps now, so they definitely seem to run better on the 4.
I would say, as of right now, there isn't a specific reason to upgrade, but I have a feeling that were I to get another 3 I would be kicking myself after a few more software iterations on the 4.