Number of diastereomer of the given compound is:

Number of chiral C-atoms = 3, (n= 3, odd number and terminal groups are same).Number of total isomers = 2n- 1 - 23 - 1 = 4.Number of optically inactive (meso forms) = 2n−12=1.Number of optically active forms = 4 - 1 = 3Hence number of diastereomers = 3.II and III are same. Check after rotating to 180°..'. Diastereomer pairs are1. I and II or III 2. I and IV3. II or III and IV