You can download WhatsApp for your Nokia 8110 4G if your device is running software version 16.xx.xx or newer. You can check your software version by navigating to Settings > Device > Device Information. To update your device, head to Menu > Settings > Device > Device Information > Software update.
To download the WhatsApp application, press Store from the apps menu. Scroll to the side and select Social > WhatsApp, then press Select > Get. The app download will then begin.