Common problems with iPad 4 chargers are that they charge slowly or that they simply stop working. These errors are most common when using a charger that isn't Apple-certified, but they can also occur when using an official Apple charger.
Keeping the connector at the bottom of the iPad and the USB cable clean can help prevent problems. If a user is experiencing connectivity issues, Apple recommends trying a new USB port, USB cable, or power adapter. Restarting the iPad 4 while connected to a wall adapter sometimes fixes slow charging issues.
If the iPad still won't charge, a new charger may be required.