B = [4, 3, 2, 1]

def a(x, y):
    return list(set(x) - set(y))
a(A, B)

def b(x, y):
    return [A for A in x if A not in y]
b(A, B)

def c(x, y):
    return [_ for _ in x if _ not in y]
c(A, B)
Python- Find unmatched values from multiple lists