So as you are well aware, our options for hardy seedless grapes are limited.
Honestly, I would plant a 2nd vine of Somerset because they are so delicious but not super vigorous nor productive.
Trollhaugen is fairly good. (I think I have one that was mislabeled as a green grape). Ripens mid September. Suffers from some shatter. But I prefer the size and flavour of Concord or Bluebell but those have seeds.
Between Reliance and Vanessa I would go with Vanessa. Reliance tastes a bit better but Vanessa seems a bit more hardy in my area (Reliance is in a bit wetter area of the row so that might be a bit more vulnerable). BUT both of these are borderline for zone 4 and need to be laid down each fall and you will still have dieback.
For seeded I like the flavour of Concord better than Bluebell plus the skin of Bluebell gets Tougher if you let it hang on the vine too long. However Bluebell is much more hearty then Concorde and ripens earlier.
I have not yet tasted Swenson Red, Swinson White, Kay gray, Roland, or Brianna but I am hoping in the next year or two to have grapes.
So to summarize from the grapes that I have tasted so far if you don’t want a second vine of Somerset I would try Trollhaugen. If you are willing to risk die back I would consider Vanessa or Reliance as a trial.