distance of a two dimension parity check code

Question

Is there any hint to find that d(C) = 4 where C is a two dimensional parity check code?

I know the first step is to show that $d(C)\geq4$, then by finding an example to show that $d(C) = 4$. But I am unable to show that $d(C)\geq 4$, is there any hint?