Cyclic compound stereoisomer
By definition, stereoisomers are the compounds which have same connectivity of atoms but different spacial orientation of atoms and two stereoisomers do not have same orientation from any point of view. 
There are different possibilities can a molecule of cylclopentane-1,2-diol can have:
For simplicity, I have made two hydroxyl ($\ce{OH}$) into two methyl groups.
But by definition you can cross out two structures because there exist a vantage point from which two molecules look the same (rotate molecule 180 out of plane and then rotate it slightly in plane, you get 2 pairs of identical molecules i.e 1,4 and 2,3. Hence, there are only two stereoisomers possible for cylopentane-1,2-diol.
Hope that helps.