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The article below has troubleshooting steps for your iPhone 4S under the "Buttons and Switches" section.
iPhone: Hardware troubleshooting
Hope that helps,
I'm having the same issue with my 4s currently as well (just made an appointment to go to the Apple store today and I'm hopint for a iPhone 5 replacement since I'm sure they don't carry the 32GB iPhones in the store nor do I think they will just repair the phone).
As far as Davids suggestion I just checked out the link and I blieve the issues that you and myself are having aren't softwar related as they seem to be a hardward issue with the physical button itself.
Currently Being ModeratedMay 8, 2013 10:21 AM (in response to Peter Gillespie)
From my understanding they will replace with the same phone if they have them in stock. I had a 3G that was replaced work a 3Gs because they didn't have my phone (GB wise) in stock anymore. So if they don't have the 4s in stock in 32GB then they will replace with a 5 since I doubt they will repair the phone.