I have not seen any 4 car trains but they should absolutely do that. There is no need for such capacity at certain times of day.
Longer headways means higher shop time and fewer drivers. I would certainly prefer 4 car trains at shorter headways with the same throughput of cars as 6 car trains in the low ridership hours.
Ridership could likely fit in a single car or at most 2 cars during much of the off-peak hours with my observations being only perhaps 20% of seats taken in typical cars.
Longer headways means higher shop time and fewer drivers. I would certainly prefer 4 car trains at shorter headways with the same throughput of cars as 6 car trains in the low ridership hours.
Ridership could likely fit in a single car or at most 2 cars during much of the off-peak hours with my observations being only perhaps 20% of seats taken in typical cars.