= Solution
An incoming <fermion> contributes a <Dirac spinor> $u_s(p)$ and an outgoing <fermion> contributes its <Dirac adjoint> $\bar u_s(p)=u_s^\dagger(p)\gamma^0$. The label $s$ specifies a <spin> state; $p$ is the on-shell <four-momentum>. Incoming and outgoing <antifermions> instead contribute $\bar v_s(p)$ and $v_s(p)$.
The exchanged <boson> is a virtual <photon>, represented in the <Feynman rules> by the <photon propagator> $-ig_{\mu\nu}/(q^2+i\epsilon)$ in <Feynman gauge>. Its <momentum transfer> is $q=p_1-p_3=p_4-p_2$; unlike an external real <photon>, it need not satisfy $q^2=0$. Each <Feynman vertex> contracts a <gamma matrix> with this <propagator> and contributes $-ie\gamma^\mu$ in a conventional <QED> sign convention. The <coupling constant (physics)> $e$ sets the interaction strength, with <fine-structure constant> $\alpha=e^2/(4\pi)$. One may reverse the sign of the <photon> field and both vertex signs without changing the result. The <gamma matrices> obey $\{\gamma^\mu,\gamma^\nu\}=2g^{\mu\nu}I$, with <Minkowski metric> $g=\operatorname{diag}(1,-1,-1,-1)$.
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