My best answer would be:

HCl]4[O2] / [H2O]2[Cl2]2

Is this correct?

5 answers

for what problem?
Taking note of the equilibrium

4HCl(g) + O2(g) ----> 2H2(g) + 2Cl2(g).

What would be the the equilibrium law expression for the balanced chemical equation?
No. It's the product of the PRODUCTS of the reaction divided by the product of the REACTANTS, each raised to a power indicated by its coefficients in the balanced equation.
I want to apoligize. The equilibrium is

4HCl(g) + O2(g)--->2H2O(g) + 2Cl2(g)

Not

4HCl(g) + O2(g) ---> 2H2(g) + 2Cl2(g).
And the equilibrium STILL is the product of the products of the reactants, divided by the product of the reactants, each raised to a power indicated by the coefficient in the balanced chemical equation. The products of the reaction are H2O and Cl2 in your corrected equation. The reactants are HCl and O2.