Is there an app similar to the Siri app for the iPhone 4?
It depends on what you mean by “similar”; but, there are quite a few, not to mention Cydia jailbreak options. Just to name some for iOS, there are Evi [1], Speaktoit [2], and Voice Actions [3].
[1] http://itunes.apple.com/us/app/evi/id463296609
[2] http://itunes.apple.com/us/app/speaktoit-assistant/id491854246
[3] http://itunes.apple.com/us/app/voice-actions/id422037126
If you mean Android apps similar to Siri, then there are even more options available. I suggest checking out my Meta Guide webpages, “100 Best Android Assistant Videos” [4] and “Best Chatbot Apps” [5].
[4] http://www.meta-guide.com/home/about/videography/100-best-android-assistant-videos