[faizan]

Why order of reaction is not more than three?

[juanrga]

Why order of reaction is not more than three?

Fourth order reactions have been reported.

[Jorriss]

As mentioned above, there are orders of more than three.

The reason elementary reactions usually don't occur with more than three species is just a matter of probability. An elementary reaction having an order of three or higher means three molecules have to interact at one time, with just the right orientation and energy. It is very rare that four, or five or six or more molecules will all hit each other at the same time in such a way that they will react to form a new molecule.

[juanrga]

As mentioned above, there are orders of more than three.

The reason elementary reactions usually don't occur with more than three species is just a matter of probability. An elementary reaction having an order of three or higher means three molecules have to interact at one time, with just the right orientation and energy. It is very rare that four, or five or six or more molecules will all hit each other at the same time in such a way that they will react to form a new molecule.

The OP is asking about order of reaction, which is not the same than molecularity. Fourth order reactions are experimentally known.

So far as I know reactive collisions of four molecules are not experimentally known and theoretically are very very dubious; as you admit, the probability of such an event is close to zero.