A function f(x_1, \dots, x_n) from a product domain \mathcal{D}_1 \times \cdots \times \mathcal{D}_n to an abelian group \mathcal{G} is a direct sum if it is of the form f_1(x_1) + \cdots + f_n(x_n). We present a new 4-query direct sum test with optimal (up to constant factors) soundness error. ... more >>>