why they say : fifth column ? why "fifth" and which are the rest four ?

Answer 1

This dates from 1936 and the Spanish Civil War.

In a propaganda broadcast Emilio Mola, a nationalist general, claimed that he had four separate columns (of troops) advancing on the capital, Madrid, and an additional "Fifth column" already in the city.
This referred to spies, saboteurs, and sympathisers ready undermine the city *from within*.

The term became widely used of people seen as hidden enemies or subversive elements within of a state, and is still current.
"We have long warned on these pages that the peace movement is not about peace, that it is a fifth column communist movement to destroy America and give victory to our totalitarian enemies. Now this Fifth Column is preparing to move into action to attempt to defeat America in its war against Saddam." 2003 (URL)
Quoted for usage only, not for political comment.

Answer 2

Emilio Mola Vidal, a Nationalist general during the Spanish Civil War (1936-39), originally coined the term. As four of his army columns moved on Madrid, the general referred to his militant supporters within the capital as his "fifth column," intent on undermining the loyalist government from within.