Do not waste time downloading “iOS 4.2.1 WhatsApp IPA” files from sketchy sites. Even if you find one, WhatsApp’s servers will reject it. Your account safety is not worth the risk.
WhatsApp officially dropped support for iOS 4.x back in 2016. The last compatible version was WhatsApp 2.12.x. Even if you find an old IPA file, WhatsApp’s servers will reject the connection because the security certificates and API requirements are outdated. Ios 4.2.1 Whatsapp Ipa Download