“Download an older version? The current version requires iOS 12.0 or later, but you can download the last compatible version.”
Q: Can I use alternative messaging apps on my iPhone 4s with iOS 9.3.6? A: Yes, there are alternative messaging apps, such as Facebook Messenger, Telegram, and Signal, that are compatible with your device.
I understand you're looking for a way to get WhatsApp on an (the final version for that device).