If you must use an iPhone 4 or another iOS 7.1.2 device, your options are extremely limited: WhatsApp Web : You can try to access WhatsApp Web
Upon opening the app, WhatsApp requires a phone number verification. The app will immediately display an error stating that the version of WhatsApp has expired or that a network error has occurred. Because the communication protocols have changed entirely, an iOS 7 client cannot talk to modern WhatsApp servers. 4. Risks of Attempting Installation whatsapp ipa for ios 7.1.2
If you must use an older device for messaging, consider these options, though their support for iOS 7 is also extremely limited: WhatsApp On IOS 7.1.2: What You Need To Know If you must use an iPhone 4 or another iOS 7
This report investigates the feasibility of installing and operating WhatsApp Messenger on Apple devices locked to (such as the iPhone 4). While .ipa files (iOS application archives) for older versions of WhatsApp exist in web archives, the service is completely non-functional due to server-side enforcement by Meta. 2. Current Compatibility Status 2. Current Compatibility Status