Is Siri available on the iPhone 4?

No, for iPhones it's only on the 4S and 5

No. Only iPhone 4S, 5, ipad mini, and ipad 3 and 4, and iPod touch 5G

iOS 6: Which software features does my iPhone, iPad, or iPod touch support?

http://support.apple.com/kb/HT5457

 Cheers, Tom 😉