Websites like are the industry standard. They pull direct links from Apple’s servers. Select "iPhone" -> "iPhone 4S". Green entries are "Signed" (can be installed via iTunes).
If you are looking to push the limits, the jailbreak community offers tools to modify your firmware. The primary jailbreak for iOS 9.3.6.
Many enthusiasts choose to "skachat" (download) iOS 6.1.3. This is widely considered the best firmware for the 4S because of its skeuomorphic design and lightning-fast speed.
The iPhone 4S is a "forever phone" in the eyes of the modding community. Whether you stay on the modern-ish iOS 9.3.6 or retreat to the nostalgic speed of iOS 6.1.3, the firmware you choose defines the soul of the device.
When looking for IPSW files (the format for iPhone firmware), reliability is key to avoid malware or corrupted files. 1. Official Repositories
The 30-pin connector must be high quality; cheap cables often fail during firmware writes. The Verdict
Apple still signs the OTA (Over-the-Air) manifest for iOS 6.1.3 for the 4S, making it one of the few devices that can be officially downgraded without "blobs."
Learn the button combination (Power + Home) to put the device into Device Firmware Update mode if it gets stuck.