Q: siri

make sure its turn on settings - general - siri - on.

then hold down the home button until siri shows

It is an iPhone 4s right, not just an iPhone 4?

As Stated... make sure Siri is on

Settings > General > Siri > Siri On.

To use Siri: Hold the home button for 3-5 seconds and speak to it.

About Siri...

http://support.apple.com/kb/HT4992

Siri...

http://www.apple.com/iphone/features/siri-faq.html

iPhone 4 does not have Siri.

Siri does not even appear in settings, so you can't turn it on or off, it's just gone.  I have reset and restored my phone twice, and no Siri.  I am going to request a replacement.

Do you have an iPhone 4S, not an iPhone 4? Check here -->    https://selfsolve.apple.com/agreementWarrantyDynamic.do

Is it set to a language that Siri supports?

Have you installed a corporate email account from a company that does not permit Siri (such as IBM)?