You can't re-install iOS 8.4.1 onto your iPhone as it's no longer being signed by Apple.
You can't install the original firmware, you can only install the latest firmware; iOS 9.2.1 at the moment.thats fine , I want to install the original firmware
All you have to do is is go to Settings>General>Software Update and the update will be there ready for downloading OTA. If you want to update through iTunes, use the update button on the summary page when your iPhone is connected to iTunes.Where can one download the latest ipsw for an iPhone 4S? Looking for 9.3.1.