So looking at http://www.cpu-world.com/Compare/416/Intel_Xeon_E5-2666_v3_vs_Intel_Xeon_E5-2680_v2.html the processor in the c4 is a newer generation, which has CPU speed benefits as Mike Lowe points out, but the potentially more significant points may be the added support for DDR4, if Amazon are taking advantage of this, and the new instructions, if the underlying zip library is able to use them.
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So looking at http://www.cpu-world.com/Compare/416/Intel_Xeon_E5-2666_v3_vs_Intel_Xeon_E5-2680_v2.html the processor in the c4 is a newer generation, which has CPU speed benefits as Mike Lowe points out, but the potentially more significant points may be the added support for DDR4, if Amazon are taking advantage of this, and the new instructions, if the underlying zip library is able to use them.
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See (second) update above about the use of c5.4xlarge in comparison to c4.4xlarge.