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 , Speaktoit , and Voice Actions .
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”  and “Best Chatbot Apps” .