Total number of stereoisomers of cyclobutane-1,2,3,4-tetracarboxylic acid

Question

Find all stereoisomers of:

My attempt

I got four isomers, and none of them seem to be optically active as they either have a plane of symmetry or a point of symmetry:

However, my sir keeps insisting there are five isomers. Could anybody help me identify the fifth one?