Google is phasing out old Android versions: new updates published on the store must support new SDKs and deprecate older ones.
People with old Android versions, like 4.3, might not be able to run Hybrid Assistant anymore, if updated from the Play Store.
For those still using old Android versions, here is the last APK that still supports them.
You might have to remove the current version before installing this one.
Do not update from the Play Store after installing.
The APK is provided as-is, with no support and no future updates: if you want the latest Hybrid Assistant version, get a new Android device.