Needed a nudge from this forum to get how to pair thematic items, but all done now! Very enjoyable.
I can see how there’s confusion about how many pieces of artwork, it’s not completely unambiguous, but suddenly the most logical is the following. We have four pairs, so to fulfil the saying and respect the pairings, we need four pieces of artwork, one for each pair.
(I know the saying doesn’t show up anywhere explicitly, but it’s too appropriate not to be taken into account, I think)