You don’t own an iPhone 4S, but you would still like to have Siri on your iDevice. We have a few legal solutions for your problems.

Spire
This is an app that can be downloaded from Cydia (so yes, you do need a jailbroken device).
The installation will take approximately 100MB directly from the Apple servers (and yes, it is legal), so make sure you are connected to a wireless network.
This method is by far the most advanced, since it is actually Siri voice recognition that will be installed on your device. However you will not be able to actually use the Siri as if at an iPhone 4S. Siri sends the data to the Apple servers directly, and receives back the information it needs. That can only happen (for now) with an iPhone 4S. The solution is to have your own Siri Proxy Setup, or use any available online hosts as shown below (Beware. By using these connections, the information sent might not be secured or protected. For instance, if you ask Siri to send a message, that request will be sent to the chosen server to compute. So by then it could be “read” or “copied”, thus endangering the system’s security).
Below you can find a few FREE proxy server hosts. However these have proven to be the most stable ones, others can be found online:

What if your device is not jailbroken? Fortunately we have an answer for you. There are a couple of solutions worth to mention from the App store.
Vlingo
By far the most interesting (and with good reviews) is Vlingo. It accepts orders to send SMS’s, E-mail, to search Maps, make Social Updates, Web Search and it can also Voice Dial. All of this on a free app.
Of course it has a few drawbacks. It doesn’t rise to the Siri level on voice recognition. Vlingo doesn’t speak back or reply when spoken to. It is highly inaccurate on loud environments, and sometimes it requests to tap when we’re finished talking. But most of the times it is reliable, and most importantly, it is for free. Besides, you can install it in your older iDevices, as iPhone 4, iPhone 3GS, and even on an iPhone 3G (yes, I tested it, and it works).

Voice-Actions
Another virtual assistant app is “Voice-actions”. Voice-actions is a paied app (3.99$). Besides doing everything Vlingo does, Voice-Actions also allows you to set reminders and alarms, ask questions, play music and even get instant translations. With these enhancements it is said to be the best alternative for Siri, available to the older models. It is indeed an alternative worth considering if you’re not planning to upgrade your iPhone soon.
Given the high demand of Siri on older devices, it’s will just be a matter of time to see more and more of Siri like apps appearing in the iPhone community.