Higher order reactions (order > 3) are rare because they require multiple reactant molecules to collide simultaneously in the correct orientation with sufficient energy. Let's break this down step by step:

Step 1: Understanding Reaction Order
The order of a reaction is the sum of the exponents in the rate law. For example, a reaction with rate = k[A][B][C][D] would be 4th order. This implies that the rate depends on the simultaneous collision of four molecules.

Step 2: Probability of Multi-Molecular Collisions
For a reaction to occur, the reacting molecules must collide. The probability of two molecules colliding is much higher than three, and the probability of four or more colliding at the same time is extremely low. As the number of molecules required increases, the chance of such a simultaneous collision decreases drastically.

Step 3: Orientation and Energy Requirements
Even if multiple molecules collide, they must be in the correct spatial orientation and possess energy equal to or greater than the activation energy (E_a). The more molecules involved, the harder it is to satisfy both conditions simultaneously.

Step 4: Why Other Options Are Incorrect
- Shifting equilibrium due to elastic collisions: Elastic collisions conserve kinetic energy and don't affect equilibrium; they are not the reason.
- Increase in entropy and activation energy: Entropy actually increases when more molecules are involved (favorable), but the probability issue dominates.
- Loss of active species: This isn't a general reason; active species are more relevant in chain reactions.

Final Answer: The correct reason is the low probability of simultaneous collision of all the reacting species.

Related Topics & Formulae

Collision Theory: Reactions occur when molecules collide with sufficient energy and proper orientation. The rate constant k is given by:
$k=PZ{e}^{-\frac{E_a}{RT}}$
where P is the steric factor, Z is the collision frequency, E_a is activation energy, R is gas constant, and T is temperature. For higher order reactions, P becomes very small.

Reaction Mechanism: Most higher order reactions occur through a series of lower order (e.g., first or second order) elementary steps. The overall order is the sum of the orders of the slow step and preceding steps.