So, it states that when transmuting 4 rivens into a brand new one, it will always be one of the types of those rivens used, so 4 rifle rivens will always result in a new rifle riven.
What if you transmute a single rifle riven and 3 archgun rivens? Is it a 25% chance that you will get a rifle riven and a 75% chance to get an archgun riven, or is it 50/50 to get either?