Taken from R&T’s July 1979 issue, The 604 was essentially a new, larger body with a V6 engine on the underpinnings of the 504. That was a good foundation upon which to move upmarket.
What really signaled the end for the 604 was the 505, which was smaller yet managed to remain comfortable and roomy enough to tick the right boxes. The 604 thus became somewhat of an overkill. Still a very competent car.