Apple introduced “App Expiration” via provisioning profiles. Even if you sideload an old IPA via Cydia Impactor or a similar tool, the certificate tied to that specific build (often an enterprise certificate or a jailbreak certificate) has almost certainly been revoked by Apple.
iOS 4.2.1 is the final stop for the iPhone 3G and the original iPad. Modern app development has left this firmware far behind. Ios 4.2.1 Whatsapp Ipa Download