Four- and five-body simultaneous collisions are very improbable events. A reaction in which four or five molecules interact is much more likely to occur in a series of steps, in which two reactant molecules first collide and form an intermediate substance, this substance collides with the next reactant molecule, and so on.
The observed rate law is a summary of all of these steps, and may depend on reactant concentrations in a complicated way. The form of the experimental rate law is the first step on the way toward unscrambling the actual mechanism of reaction, but it does not give us the entire story.
Only for simple, one-step reactions will the order of the rate law necessarily agree with the coefficients of the balanced equation. This occurs almost exclusively with first-order decompositions and with those relatively rare second-order collisions that are uncomplicated by further reactions.What we have just said applies to rate expressions, but not to equilibrium-constant expressions. The exponential coefficients in the equilibrium-constant expression do match the coefficients in the balanced chemical equation. The equilibrium-constant expression for the HI reaction is