= Solution
In the <defining representation>, the diagonal element of the <Cartan subalgebra> acts on the four standard <basis vectors> with <eigenvalues> $r,s,-r,-s$. Therefore
$$
\boxed{\operatorname{wt}(V)=\{L_1,L_2,-L_1,-L_2\},\qquad\text{each with multiplicity one}.}
$$
Their <weight diagram> is the four-point cross below.
\Image[/past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2005/iii/paper-2-sp4-defining-weights.png]
{title=The four defining-representation weights of sp4}
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