I think four flashes indicates a faulty pre-tentioner.

They had big problems with the yellow connectors under the trims by the outside of each seat.

You want to try disconnecting these and putting a small amont of vaseline (ooh er) in the connector block and reconnecting.

Make sure you have the battery disconnected for about 10 - 15 mins before you do this or you could get an air bag in the face.

HTH.