The Gita has related the four orders of society, not to a fixed outward rule governed by the accident of birth in a particular strata of society, but to the inner capacity of the individual and the turn of his natural tendencies. This inner capacity and these tendencies relate to the predominant balance of the play of the three Gunas in each individual. As an individual evolves and grows in capacities, and as the evolutionary development moves him systematically through a predominant balance of tamas, rajas or sattwa, we see that the natural interest and work to be done by the individual mirrors that balance, which is not fixed for all time and through multiple lives, nor inherited, but something that is fluid with the development of the inner truth within the individual. it should also be remembered that the three Gunas are always at play and thus, for any individual, there will be some amount of action of all three of them, despite an overwhelming predominance that may be seen for one or another of the Gunas in a particular life.
Sri Aurobindo extrapolates on this point: “It is true that in this birth men fall very largely into one of four types, the man of knowledge, the man of power, the productive vital man, the man of rude labour and service. These are not fundamental divisions, but stages of self-development in our manhood. The human being starts with a sufficient load of ignorance and inertia; his first state is one of rude toil enforced on his animal indolence by the needs of the body, by the impulsion of life, by necessity of Nature and, beyond a certain point of need, by some form of direct or indirect compulsion which society lays upon him, and those who are still governed by this Tamas are the Sudras, the serfs of society who give it their toil and can contribute nothing or very little else in comparison with more developed men to its manifold play of life. By kinetic action man develops the rajasic Guna in him and we get a second type of man who is driven by a constant instinct for useful creation, production, having, acquisition, holding and enjoying, the middle economic and vital man, the Vaishya. At a higher elevation of the rajasic or kinetic quality of our one common nature we get the active man with a more dominant will, with bolder ambitions, with the instinct to act, battle, and enforce his will, at the strongest to lead, command, rule, carry masses of men in his orbit, the fighter, leader, ruler, prince, king, the Kshatriya. And where the sattwic mind predominates, we get the Brahmin, the man with a turn for knowledge, who brings thought, reflection, the seeking for truth and an intelligent or at the highest a spiritual rule into life and illumines by it his conception and mode of existence.”
Sri Aurobindo, Essays on the Gita, Second Series, Part II, Chapter 20, Swabhava and Swadharma, pp. 504-505