Doesn't mean there were no SSDs during ep4. Let's not forget by Ep6 there wrre "lots of command ships" according to Han...
I haven't been to the imperial retreat in a while, but I thought the trio in orbit were an SSD flanked by SDs.
Edited to add: Oh look, this is the post that makes me a wampa. *runs off to kill and eat a cu pa ingame*