Seibert’s class IV defect is
Seibert’s  Classification
Class 0 defects- no defect
Class I defects- faciolingual loss of tissue width with normal ridge height.  
Class II defects- loss of ridge height with normal ridge width.
Class III defects- a combination of loss in both dimensions.
Faciolingual loss of tissues with normal ridge height
Normal ridge width with loss of ridge height
Loss of height and width
None of the above