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Yes, I really think maximum 4 mana for Delwyn.
With untamed cards is now a new dragon summoner with -1 Melee for 3 Mana.
If consider that Delwyn is a promotion card it's even the question if not for 3 Mana.
But I think it's unrealistic with 3 Mana, so 4 Mana would be an compromiss.