Four sets is generous for Trondheim-Bodø, but not enough for two Oslo-Bodø daily as there is no spare.The ones ordered are for the Trondheim-Bodø trains, making it possible to retire the Di4s. Not sure if four trains are enough to also use them on other lines.
You could manage a 4-day cycle running Oslo - (night) -Trondheim - (day) - Bodø - (night) - Trondheim and vv, giving a whole day at O and T for maintenance, but that would break down as soon as a set had to be taken out of traffic. A 3-day cycle involving a day train Trondheim-Oslo-Trondheim would involve a tight turnround in Oslo if the 4th set was stopped, so might be better.
Mind you, if I were a business traveller from Trondheim I might turn my nose up at a train that had come overnight from the north. It will be interesting to see whether the through workings every happen.