= Solution
Here $V_m$ is the inside-minus-outside <membrane potential>, $V_K$ is the <potassium> <Nernst equilibrium potential>, and $\bar g_K$ is the maximum <potassium> conductance, or conductance per area if $I_K$ is a <current density>. The factor $n\in[0,1]$ is the permissive-state <probability> of an activation gate. Four <independent permissive gates in an ion channel> give the open fraction $n^4$, hence
$$
I_K=n^4\bar g_K(V_m-V_K).
$$
The model assumes identical independent gating units, additive currents from many <ion channels>, an approximately ohmic open-pore current, and fixed ionic activities setting $V_K$. Outward positive <electric current> is obtained for $V_m>V_K$. The original delayed-rectifier term omits rapid <ion-channel inactivation>. The fourth power is a successful phenomenological gating description, rather than a deduction of the molecular structure from this equation alone.
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