It is said that once a quarrel started between Brahmaa Jee and Vishnu as to who was the greater between them. The quarrel went unsettled for ten thousand Divya Varsh (Divine years). Then suddenly there appeared a pillar of flame of light between them and both were surprised to see this third thing appeared.

They were curious to know as what it was, so Brahmaa Jee told Vishnu that he was going to find where its top was, so he assumed the form  of a white swan and flew upwards; and Vishnu thought to find its bottom or root or the beginning so He assumed the form of a white boar and went downwards. Later Vishnu appeared saying that He could not find the pillar's bottom. A few moments later Brahmaa Jee also appeared saying that he found the top and brought with him witnesses, one of which was the Kaamdhenu cow.

Now appeared Shiv before them and said - "Brahmaa told all lie saying that he found the top while he did not find any, so he shall not be worshipped at all on Prithvi; but because Vishnu admitted the truth that He did not find the bottom, he will be worshipped by everybody. That is the reason why we do not have any temple to worship Brahmaa Jee, and Vishnu is worshipped everywhere. Besides, he cut one head of Brahmaa from which he told lie. After this incident Brahmaa had only four heads.

Here is the astrological meaning of the story also. Brahmaa is the Polar Star or North Star, while Vishnu is the Sun whom we call Soorya Naaraayan. The Pole Star is high in the sky above the North Pole. If we look down from the Polar Star, we see the earth, also known as the cow, which is Kaamdhenu. If we look towards the sky from the Earth or North Pole, the Polar Star is apparently seen at the top of the sky, but the truth is that the Lunar Pole or the center of the Lunar orbit ie Barry center is the actual top, which is Shiv himself.