In the book, The Scourge of Salafi'ism you mention that the four Math-habs are as old as Islam and had existed from the time of Rasulullah (sallallahu

Answer

The Shariah of Islam is the product of Wahi (Revelation) from Allah Ta'ala. It was perfected and finalized during the very lifetime of Rasulullah (sallallahu alayhi wasallam) and the Sahaabah. The later Fuqaha (Ulama of Islam) merely codified it into a systematic order for the benefit of posterity. They did not introduce new laws into the Shariah.

Consider the issue of raising the hands in Salaat. Whilst the followers of the Shaafi' Math-hab practise Rafa'Yadain (Raising the Hands) during Salaat, the Hanafis do not. Both ways were practised by Rasulullah (sallallahu alayhi wasallam). Another example: The Shaafis recite Qunoot during Fajr Salaat while the Hanafis do not. Both ways were practised by Rasulullah (sallallahu alayhi wasallam).

Once a Sahaabi recited Surah Ash -Shams. Hadhrat Umar (radhiyallahu anhu) hearing the recitation grabbed the Sahaabi and dragged him to Rasulullah (sallallahu alayhi wasallam), claiming that he was interpolating in the Qur'aan with the 'erroneous' recitation. Rasulullah (sallallahu alayhi wasallam) instructed Hadhrat Umar (radhiyallahu anhu anhu) to leave the Sahaabi. Then Nabi (sallallahu alayhi wasallam) ordered Hadhrat Umar (radhiyallahu anhu) to recite Surah Ash-Shams. After he recited it, Rasulullah (sallallahu alayhi wasallam) commented that he had recited it correctly. Then he ordered the other Sahaabi to recite it. After he had recited it the way he had done before, Rasulullah (sallallahu alayhi wasallam) commented that he too had recited it correctly.

From this example it is clear that even Hadhrat Umar (radhiyallahu anhu) was not aware of the other correct form of recitation. From these couple of examples you should understand that the differences existed during the very age of the Sahaabah. The Sahaabah settled in the different parts of the expanding Islamic Empire, and each one taught the Deen as he had understood it and seen Rasulullah (sallallahu alayhi wasallam) practising it. Furthermore, the Sahaabah themselves interpreted certain sayings of Rasulullah (sallallahu alayhi wasallam) differently, and some used their Ijtihad to formulate rulings on issues on which Rasulullah (sallallahu alayhi wasallam) had not issued clear directives. These Sahaabah taught the Deen to their Students among whom were the great Mujtahid Imaams. They disseminated the Deen as they had acquired and practised it. Thus the differences became entrenched in the Ummah, and were categorized in the Math-habs.