The external Power Packs are minimum requirements recommended in the manuals. Basically all Power Packs are backwards compatible (i.e. Canon CP-4 to CP-3 and CP-2 or Metz P76 to P50 and P40) but for instance the CP-2 is less powerful than CP-4, so a flash powered by CP-2 that is original intended for CP-4 might (most likely will) not perform as expected. Other way around you can use CP-4 instead of CP-2.
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