U4 coils are pencil coils with dual spark / wasted spark technology. It is also known as a stick, mushroom or coil on plug design. In this version the coil supplies two spark plugs with high voltage.
The coil is attached directly onto the spark plug; an additional high voltage ignition cable connects to the corresponding second spark plug.
A different name for these ignition systems is "Semi-direct ignition".
Example of this type: U4022, used by Kia