If you truly need WhatsApp, consider upgrading to an iPhone 5 or newer that can run iOS 10 or later. Refurbished iPhone SE (1st gen) costs ~$50–70 and runs iOS 15.

, which allows the phone to run unsigned or modified IPA files. Sideloading: Tools like are often used on a PC to push the IPA onto the iPhone. The "Plot Twist": The Dead End