WhatsApp will stop working on some Android phones
Another year is about to end in the next three months, which means there is another cycle of WhatsApp's end of support for some Android smartphones and iPhones. After November 1, 2021, the messaging app will not work on some of the Android versions.
WhatsApp is used to removing the support for specific Android versions. This is a common technique because developers always prefer to support the newest OS updates, and it's hard to always support old versions. It's the case of Android OS 4.0.4 and older versions: WhatsApp has announced that they will no longer support Android phones running those versions after a new WhatsApp beta for Android update.
What can you do to continue using WhatsApp on those versions? Unfortunately, you won't be able to update WhatsApp after November 1, 2021, and your current WhatsApp update is going to expire after a certain date.
You need to switch to a new Android device on a supported Android version if your device is old, so be sure to back up your chat history. Note that WhatsApp is also going to support chat history migration from Android to iOS in a future update if you're interested to switch to iPhone.