Molecule II and IV identical because. If you draw the structure of two and look the same structure from below the plane, you will get IV.
If two compounds are not enantiomerically related then they are diastereomers.
From these compounds shown, you can see that there are no non superimposable mirror image i.e no enantiomers.
I and III have plane of symmetry so they are mess compounds - optically inactive. So both of them aren't enantiomers of each other and are diastereomers.