Use WhatsApp Web via Safari on iOS 7.1.2 instead. Navigate to web.whatsapp.com in Safari. While you cannot get push notifications, you can read/write messages via the web interface without installing any IPA.
Whatsapp on my iphone 4 with ios 7.1.2 is… - Apple Community 29 Nov 2018 — whatsapp old version for ios 712 download upd
To use WhatsApp on iOS 7.1.2, you need version (or older, specifically build 2.18.80–2.19.80). Here is what you need to know about that version: Use WhatsApp Web via Safari on iOS 7