The D4 light typically indicates a problem with the automatic transmission control system. When the D4 light is blinking it means that a fault currently exists in that system. That light can be triggered by any of the monitored components, including the transmission control unit, the shift indicator switch or any of the inputs to the control unit.You didn't mention what the code was that the dealer retrieved. If you could get that information ti would be a big help in diagnosing you problem. My gut feeling is that the problem is with the shift indicator switch that "tells" the control unit that the vehicle is in Park. There is a switch called a "neutral safety switch" that will not allow the car to start unless its in park or neutral. If that switch is bad you won't be able to start the car and the light will flash.How does your husband get the car started? If he has the problem again, ask him to take the car out of Park and to try to start it in Neutral. That may overcome a problem with worn contacts on the gearshift switches.the main relay for the fuel pump that people have told you about is a condition that occurs when the car is parked in the sun an the interior temperature rises. The main relay problem will not cause the D4 light to come on. It would also be extremely unlikely that the transmission control unit went 'bad' after 15 years.The problem is most likely caused by a loose connection somewhere in the starter control circuit of the transmission control unit or a switch (range selector or neutral safety switch) that has worn out over time. Hope that helps