Calculating attribute closures:
{A}+ = {A}
{B}+ = {B, C}
{C}+ = {C}
{D}+ = {A, D}
{A, B}+ = {A, B, C}
{A, C}+ = {A, C}
{B, C}+ = {B, C}
{A, D}+ = {A, D}
{B, D}+ = {A, B, C, D} <--- Composite minimum candidate key
{C, D}+ = {A, C, D}
{A, B, C}+ = {A, B, C}
{A, B, D}+ = {A, B, C, D} <--- Superkey
{A, C, D}+ = {A, C, D}
{B, C, D}+ = {A, B, C, D} <--- Superkey
{A, B, C, D}+ = {A, B, C, D} <--- Superkey

{B, D} is candidate key

Lets check for Highest normal form:

FOR BCNF LEFT SIDE FUNCTION DEPENDENCY SHOULD BE A SUPER KEY
B->C [ B IS SUPERKEY AS IT IS A PART OF CANDIDATE KEY BD] SO IT IS IN BCNF
D-> [D IS SUPERKEY AS IT IS A PART OF CANDIDATE KEY BD] SO IT IS ALSO IN BCNF

ALL CONDITION SATISFY FOR BCNF SO HIGHEST NORMAL FORM IS BCNF.