There are two types of equilibrium constants: Kc for concentration and Kp for partial pressure. When the reaction involves gaseous substances, Kp is applied. Hence, we use Kp here instead of Kc. The Kp for this reaction is:
For a reaction with a general form of:
aA + bB --> cC + dD
The expression for Kp is
Kp = [C^c][D^d]/[A^a}{B^b]
Following this format, the answer is then
Kp = [HBr]⁴[CBr₄]/[CH₄][Br₂]⁴