Yes, that never existed (which in my eyes only ever made a transparent case an adequate choice for the speckled green FP4 – and the same for the semi-transparent FP5).
The speckled green FP4 and back cover was – until very very recently – only ever available from the Fairphone Shop, so I would speculate there weren’t that many speckled green FP4 produced from the start, so it probably would not have worked out economically (which seems to be backed up by the fact that the speckled green back cover as a spare part is more expensive than the other two).
Well. I bought my speckled green FP4 2 years ago at some third party store, not in the FP shop.
But anyway you are right: There never was a speckled green cover. And I never used a cover as I like the color of the back of my phone as it is.
And, the back cover could (or still can) be purchased as spare part from the FP shop and you could create a speckled green FP4 from a plain green one.