NOTE: This is not my work it came from ttg and i thought it should be shown here aswell
Credit to the user Lidl:
This is how to get siri on an iphone 3gs or 4, this will also work on an ipod touch 4g

*Proof*

Spoiler:

Requirments:
- A jailbroken iDevice, if you do not know how to jailbreak ios 5 untethered
- A WiFi connection on your iDevice.

Ok, now we can begin.

1. Open up cydia.
1a. Install any required updates from cydia.

2. Goto manage, then sources.

3. Click edit, and then add.

4. In the box that pops up, enter: "http://smolk.myrepospace.com/"

5. Once the source has been added, go onto it and find "Siri0us".

6. Install Siri0us, be paitient as it is an 100mb download. Once downloaded, respring.

8. Goto settings, scroll down and hit Spire.

9. In the proxy host box, Enter any of the following:
siri.cd-team.org/
97.188.76.85:443/
192.453.201:3544/
205.185.119.17:2000/
97.128.46.41:323/
97.128.73.85:442/
Note: These are proxy siri servers which many people are trying access, they will be VERY slow. If you want a fast connection to siri, look for a paid proxy.

10. Click return, then go back to the main settings list, goto general, then hit siri, Turn it on and choose your language.

11. Test it - Close any apps you have running, then hold down the home button, siri should pop up
NOTE:Credit Given Where Due