Appliance Repair Questions? Ask a Technician for Answers ASAP
Try cleaning out the filter at the bottom of the machine behind the flap.
Turn the machine off for a few minutes at the mains power, back on and try again.
I hope that helps, Best Regards,
Try leaving it off for about 30 minutes so that it fully discharges, then you should be able to open the door, then try again.
It might be that you've now cleared the blockage and E04 will go.
The following article describes some of the possible causes of an E04 error (for Hoover, but same for Hotpoint I believe):
Probably best if an engineer looks at this, but if you are going to investigate please ensure the power is turned off at the mains socket.
Do you require any further assistance?
If not, I hope my answer was helpful and if so a positive star rating would be much appreciated.