iPhone 4 issues with updating whats app
It is difficult to know the answer as to why this happened now and not before.  Fact is that the WhatsApp app you have installed right now had been downloaded with the Apple ID displayed to you.  What I would do is to uninstall, then re-install the app from the App Store using YOUR Apple ID.

Speculation is that you installed the app using iTunes from a different computer or someone (maybe a friend of yours?) logged themselves in with their Apple ID on the device to do so.

IMPORTANT: Any app you uninstall will also remove its settings and contents.  So, if the messages you have inside are not stored elsewhere (e.g. iCloud or on WhatsApp's servers, in case they store such items there), you will lose them.