Apple is not likely to release an iOS version that enables the use of Siri on an iPhone 4 or an iPad 2. According to CNET, the processor in an iPhone 4 lacks the technology that enables Siri to distinguish the speaker's voice from background noise.
However, Tech Crunch speculates that not releasing Siri to older models is part of Apple's strategy to entice customers to upgrade their devices to ones that support Siri. Rather than being concerned about the quality of an interactive voice feature, this idea suggests that Apple is just trying to maximize profits.
An iPhone 4 already has a Voice Control feature that can make phone calls and control music, just as Siri would. This feature is accessed by holding down the home key.
Alternatives to Siri exist for the iPhone 4. For example, Dragon Go, an app that can be downloaded to an iPhone 4, functions similarly to Siri in that it recognizes speech and can carry out requested functions. Another similar app supported by iPhone 4 is Google Search, which allows users to use spoken language to ask Google to search for information. Vlingo is an another iPhone 4 app that transcribes speech into texts, emails and tweets.