According to AppAdvice, Apple is internal testing Siri on the iPhone 4.

Apple is still considering releasing Siri on the iPhone 4 and even went as far as installing a special build of iOS 5, that includes Siri, on a number of Apple employee’s iPhone 4 handsets recently to test it out for a few days.

We recently reported that Apple may be bringing Siri to OS X Lion early next year but making it backwards compatible with the iPhone 4 seems to be a bit of stretch seeing as its a great reason for users to upgrade.

Millions of iPhone 4 users should keep their fingers crossed.