The iPhone 4s is permanently capped at iOS 9.3.6, meaning it cannot officially update to the software required by modern WhatsApp. Attempted Workarounds (Pros & Cons)
Even if you manage to install the app, you will likely face these hurdles: Verification Codes: whatsapp old version for ios 9.3.6 download
Follow these instructions precisely:
Yes. The 4s has an A5 chip (512MB RAM). WhatsApp is noticeably sluggish. Disable background app refresh to improve. The iPhone 4s is permanently capped at iOS 9