If you wish to proceed for experimental purposes, here are the potential, though not guaranteed, methods: Method 1: The "Purchased" Trick (Requires a newer device)
via the Safari browser. However, legacy browsers often lack the necessary web standards to load the modern interface properly. Device Upgrade whatsapp ipa for ios 7.1.2
While official support is gone, some users in legacy communities still attempt to bypass these restrictions. Note that these methods are and may lead to account bans or data loss. If you wish to proceed for experimental purposes,
For absolute stability on an iPhone 4 running iOS 7.1.2, stick to cellular text messaging or basic iMessage functionality. Note that these methods are and may lead
Drag and drop the downloaded WhatsApp iOS 7 IPA file into the software interface.
How to Download and Install WhatsApp IPA for iOS 7.1.2: A Complete Legacy Guide