Among the four varṇas one has to consider the presence of those without ‘colour’ also known as the ‘untouchables’ (Marriot, 2004, 379) and currently referred to as Dalits. Ghandi tried to improve the lives of them whom he renamed “children of God”’ (Marriot, 2004, 380). 'Eight verses are devoted to the Vaisya and just two to the Sudra' (Olivelle, 2005, 16) which shows the inequality between the classes in some Indian texts. The system is still followed today (Marriot, 2004, 358), for example India had a Dalit President, Kocheril Raman Narayanan.
From post-Vedic era, each of the four varṇas would include a myriad of occupational sub-classes known as jati, they ‘consist of dispersed, named networks of families’ (Marriot, 2004, 358)
Male members of the top three classes only are traditionally called ‘twice born’ (dvija) (Marriot, 2004, 359).
The second birth is part of the upanayana ceremony which usually happens between ages 3-11 depending on the varna. It is an initiation into the duties of the class and involves a ritual bath. ‘Indian governments of the twentieth century recognised thousands of jatis as units for compensatory discrimination and classified them in four categories that roughly invert the varṇas’ primordial ordering’ (Marriot, 2004, 381).
Late and post- Vedic social order introduced the four fold class, so a 4th veda was introduced which is Artharva. As you can see from the table on the four fold division of the vedas that the priest of the fourfold division of the Vedas was a Brahman, which is the first Brahman priest, as referred to in the BrhadaranyakaUpanisad: ‘In the beginning this world was only brahman’ (BĀU 1.4.11). The characteristics of the brahman priest are that he supervised everything and had mantras that correct mistakes.
A new class emerged which is sudra (which means to serve) which are slaves, prisoners of war, migrants due to the results of migration, conquest and resettling from 1000 BCE. There are specific duties for the four varṇa which literally means colour (Marriot, 2004, 357), so everyone has a colour. For example the sudra varṇa were assigned the colour black and had duties that the three other classes cannot do (Marriot, 2004, 378). And in contrast the brāhmaṇa class has a function to sacrifice and teach and their colour is white.
This Upanisad is taken from the last vedas which discusses the fourfold system. This is textual evidence of the new system, take verse 14 for example mentions ‘the priestly power’ (BĀU, 1.4. 14), so referring to the brahmana classand ‘the ruling power’ (BĀU, 1.4. 14) meaning the ksatriya class.