So if "D4" were flashing and I was driving along at 45 mph and the car shut off, even without the headlights dimming, that could still be the alt?

The flashing "D4" is tied to the tranny and even the PGM-FI as far as I've been able to find out. Is the alt? Of course, I also realize that with 183.5K miles, it's just about time for a new alt. as well anyway....