U4 indicates a top lid detected open on the AQUA units covered by this FAQ. Confirm that this is that design before applying the explanation.
The AQUA Japanese documentation links U4 to the top lid. A U4 on another product or brand is not defined by that match.
Check the lid position
With the motion stopped, see whether a garment or object is preventing the lid from closing. Remove only what is accessible and lower the lid normally. Do not press the sensor, attach magnets, or bridge its contacts in an attempt to start it.
If the lid closes completely and U4 returns, note that condition and contact support. The alert may require checking the close confirmation, but it does not identify a part that should be replaced by elimination.
UL for child lock and H during cooling are different messages. If the lid is held shut after drying, do not force it to repeat this inspection: the safe opening in its manual must come first.
Provide the model number and clarify whether the lid does not fully close or whether it closes and the alert continues.