In a recent game, I unlocked the Mark III version of the Spire Corvette. I had also captured an Advanced Starship Factory, so I had access to the Mark IV version as well. However, in the Ship Design tab under controls, I didn't have an option to create designs for the Mark IV, even though I could build them.
Am I just missing something, is it a bug, or are you just not supposed to be able to make mark IV custom designs?