What would be the correct keq expression for the following reaction that has reached dynamic equilibrium?
co(g) + 3h2(g) two arrows stacked on top of each other. the top arrow points to the right. the bottom arrow points to the left. ch4(g) + h2o(g)

So we have an equilibrium expression:
CO(g) + 3H2(g) <=> CH4(g) + H2O(g)

The equilibrium expression is the concentration of each product (excluding solids and liquids in all cases) raised to the power of their coefficient, all divided by the concentration of each reactant (again, excluding solids and liquids). Noting that brackets around a molecule indicate concentration, the expression would be:
([CH4]*[H2O])/([CO]*[H2]^3)