= Solution
With all four external <fermions> of species 1, both final attachments are possible. The middle row of the figure gives the two <tree-level Feynman diagrams>:
* The $t$ channel has lines $1\to3$ and $2\to4$, joined by a <scalar propagator> with $q_t=p_1-p_3=p_4-p_2$.
* The $u$ channel has lines $1\to4$ and $2\to3$, joined by a <scalar propagator> with $q_u=p_1-p_4=p_3-p_2$.
For a fixed ordering of external <fermion> states, the <fermionic sign> from exchanging the final identical <fermions> gives
$$
\boxed{\mathcal M=-g^2\left[
\frac{(\bar u_3u_1)(\bar u_4u_2)}{t-m^2+i0}
-\frac{(\bar u_4u_1)(\bar u_3u_2)}{u-m^2+i0}
\right],\qquad u=q_u^2.}
$$
Here $u_j=u_{s_j}(p_j)$ are <Dirac spinors>. In particular, the <scattering amplitude> changes sign when the complete final labels $3$ and $4$ are exchanged, as required for identical <fermions>.
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