The four head of Brahma symbolizes the four Vedas, the four yugas and the four directions. The four-faced head also symbolizes both the severing of all conceptualizations, and the development of altruism through the four immeasurable of compassion, love, sympathetic joy and equanimity.
Brahma is the best know expression of the concept of the cosmogenic deployment of the spatial directions is Brahma, whose four heads, facing the quarters, symbolize the directional emanation of space. He is the source, the seed, of all that is. Whereas Brahman, the unoriented and boundless immensity, the void, offers no room for existence, Brahma, the immense being, Brahman’s masculine or personified form and first affirmation, is the principle of space and time: he punctuates the void so that space and time may originate within its non-determination. He is qualified Brahman; he is identified with the hiranya garbha from which the universe develops, he is similarly identified with Prajapati, the god from who the world is formed. (Source - The Symbolism of the Stupa - Page 38 Adrian Snodgrass, Craig J. Reynolds - 1985)