If Beoplay H4 2nd Gen are connected to a USB cable, disconnect, then reconnect the cable and try again.
If connected via Bluetooth, your Beoplay H4 2nd Gen have a Bluetooth range of up to 10 meters (in direct line of sight). However, Bluetooth connectivity might be affected by other active Bluetooth devices in the nearby area (e.g. mouse, keyboard, mobile phone, laptops or 2.4 GHz routers). Try keeping the headphones away from other active devices.
If your Beoplay H4 2nd Gen are still unresponsive, find out how to reset them here.