Lots of stories about this, and it seems to be one of 2-3 possible causes.
So here's the symptoms (first, it worked last night before I went to bed):
1) E04 error code
2) I reset the heater with a power down
3) When the heater turns on, you hear a click of the relay, but nothing happens.
4) Eventually after a few minutes E04 is displayed
Now, I know that a common cause is the igniter, and dirt in the tubes, but the normal sequence of fire-up is:
1) The relay clicks
2) The fan turns on immediately
3) After 10-20 seconds (never really counted), the igniter turns on
4) It's now heating.
Does the fact that nothing happen point to the fan? It doesn't even try to start.