I actually dont know how much we are allowed to help with siri. The only way to get Siri on a 4 is to clone someones ID and use that to access a server. If im not mistaken, its highly illegal. I think.
Firstly you need to install a Siri GUI. This is assuming you're on the latest iOS 5.1.1 and jail broken.
For the Siri GUI I use Rainstone Siri. In cydia add the repo http://repo.rainstone.co.
Install from that repo Rainstone Siri for 5.1.x and Skyra1n. You'll have to reboot your iDevice after doing this.
Next you'll need to add another repo http://wordjelly.com/repo.
From there install Wordjelly control panel for 5.1.1. Once that's done go into settings, wordjelly and click install the certificate. You'll hit install and after that Siri should be ready to go.
In the wordjelly control panel there are two spaces for API codes for wolfram alpha and wunderground weather. You'll need to get these for siri to answer detailed questions and questions regarding the weather. It explains how to do so.