The 4th weekly service (Sun) only operated from Feb 8th to April 19th. Was it only planned as a short term increase or were there other reasons for it ceasing ?
I don't know why it stopped, maybe a seasonal thing. But the Sunday flight resumes on 6th September and seems to be permanent thereafter.
FI are also adding a 5th weekly flight on Thursdays, but it's currently only bookable for February to April.