However doing so frequently might damage your HOME button. So, how about leave your Home button for awhile, and subtitute it with Mic Button to call Siri?
Yes you can change the usage of HOME button to Mic button, and the button will be appearing from Notification Center.
To get that Mic button to your iPhone 4s, you are going to need a Cydia tweak namely SiriLaunch which is available for free. The app is created and developed by Jude Jean-Pierre. SiriLaunch will be placing a cute small button to launch Siri at the buttom-left your screen. But don't worry its position can be changed via Setting.app.
Once you hit the Mic button, it will launch Siri instantly like you usually do using the Home button. And are you interested to give SiriLaunch a try? Your iPhone 4s must already jailbroken, so have you done that? If you have not, Absinthe 0.4 untethered jailbreak tool is your helper to jailbreak iPhone 4s. One more thing, your device must also be running iOS 5 and higher.
And as for older models, like iPhone 4, iPod touch 4G running on iOS 5 using Spire, SiriLaunch will also work for your device. But of course, after you have found Spire Siri server, and SiriLaunch tweak is also compatible with AssistantConnect that will make your device able to run Siri without Siri proxy server.