def product(b):
    result=[]
    for i in b[0]:
        if len(b)>1:
            xx=product(b[1:])
            result+=[[i]+v for v in xx]
        else:
            result+=[[i]]
    return result
p=product([[1,2,3],[7,8],[9,4],[5,6]])
print(p)

# OUTPUT
# [[1, 7, 9, 5], [1, 7, 9, 6], [1, 7, 4, 5], [1, 7, 4, 6], [1, 8, 9, 5], [1, 8, 9, 6], [1, 8, 4, 5], [1, 8, 4, 6], [2, 7, 9, 5], [2, 7, 9, 6], [2, 7, 4, 5], [2, 7, 4, 6], [2, 8, 9, 5], [2, 8, 9, 6], [2, 8, 4, 5], [2, 8, 4, 6], [3, 7, 9, 5], [3, 7, 9, 6], [3, 7, 4, 5], [3, 7, 4, 6], [3, 8, 9, 5], [3, 8, 9, 6], [3, 8, 4, 5], [3, 8, 4, 6]]