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prominence is measured relative to the low point between the closest taller mountain.
on this island mountain 2 has no taller mountain, so it's prominence is measured from sea level
mountains 1 and 4 have the same elevation, but the saddle between 1 and 2, and 3 and 4 is different, so 1 has greater prominence. 3 is taller overall than 1 and 4, but the saddle between 2 and 3 is smaller than the 1:2 saddle or the 3:4 saddle, so 3 has the smallest prominence.