$U_1+U_2$ consists of all the elements $u_1+u_2$ where $u_1\in U_1$ and $u_2\in U_2$, right?
So, the sum is the following, or not?
$$(1,0,0,0)+(1,0,0,0)=(2,0,0,0), \ \ (1,0,0,0)+(0,1,0,1)=(1,1,0,1), \ \ (1,0,0,0)+(0,0,1,0)=(1,0,1,0), \\
(0,1,0,0)+(1,0,0,0)=(1,1,0,0), \ \ (0,1,0,0)+(0,1,0,1)=(0,2,0,1), \ \ (0,1,0,0)+(0,0,1,0)=(0,1,1,0), \\
(0,0,2,1)+(1,0,0,0)=(1,0,2,1), \ \ (0,0,2,1)+(0,1,0,1)=(0,1,2,2), \ \ (0,0,2,1)+(0,0,1,0)=(0,0,3,1)$$
@Astyx