The compound shown is 1,2,3,4-tetrachlorocyclobutane drawn without stereochemistry. Treat the ring as planar and square and consider configuration only. Which of the following statements are correct?

Each ring carbon carries one $\mathrm{Cl}$ and one $\mathrm{H}$, so the stereoisomers are the distinct up/down patterns of four chlorines on a square: all four up; three up and one down; two adjacent up and two adjacent down; and two opposite up and two opposite down. Any other pattern is one of these rotated or turned over. That is 4 stereoisomers. Each of these patterns has a mirror plane perpendicular to the ring (through two opposite carbons or through two opposite bond midpoints), so all four are achiral and the count $2^4=16$ is far too high.