When you work for a company that has iPhones as company phones you're bound to come across similar issues.
By now you could have a number of iPhone 4's that are showing performance issues if you bought them around a year from their release date.
I've had a few that have been taken back for screen repairs and battery issues so the majority of the iPhones I have don't match the original proof of purchase forms (they don't give you a new one when you get a replacement phone) and I have no way of knowing the life expectancy of the refurbished phones.
You can check if an iPhone is still under warranty by the Apple website (https://selfsolve.apple.com/agreementWarrantyDynamic.do) using the hardware serial number that can be found under Settings > General > About.
A replacement warranty lasts 90 days so it's a good idea to record when it will expire and set a reminder to check/test the phone a week beforehand.
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