Stick to Method 1.
Q: What are the risks of running WhatsApp on iPhone 4s with iOS 9.3.6? A: You may be exposed to security risks, and WhatsApp may not work seamlessly with other apps or services. whatsapp iphone 4s ios 9.3 6 download
You cannot update to newer versions. The App Store will only offer the last compatible version. Never delete WhatsApp after installing – you may not be able to re-download it if Meta pulls the legacy version entirely. Stick to Method 1