The Buddhas are ‘Teachers of Human and Heavenly Beings’ (人天教主) because they are spiritually greater than all humans and gods.
Is the so-called Four-Faced ‘Buddha’ (四面佛) really a Buddha (佛)? No. He was misnamed by some Chinese non-Buddhists to be a Buddha, when he is actually Mahā (Great) Brahmā (大梵天) (Phra Phrom in Thai), who resides in the highest First Dhyāna Heaven (初禅天).
In the Buddhist context, he was a god who mistaken himself to be the creator of the universe. However, as corrected by Śākyamuni Buddha (释迦牟尼佛), also with the name of Baka (婆迦: Crane) Brahmā, he has since realised his mistake, and is now considered a Dharma-Protector god (护法神). (More can be seen in the first link below.) Thus, to be accurate, he is a ‘four-faced god’ (四面神).
Thus, his image is smaller and placed outside main Buddha halls in countries like Thailand, which represents his supportive role in Buddhism. The four faces represent the Four Immeasurable Minds (四无量心), also known as Brahmā Vihārās (Brahmā Abodes), which are loving-kindness, compassion, rejoice and equanimity (慈悲喜舍).
With the word ‘Brahmā’ (梵) meaning ‘pure’ (清净), although Brahma gods are supposed to practise these qualities purely, looking at the world with them, the Great Bodhisattvas (大菩萨) and Buddhas are the ones who have perfected them, thus having Great Loving-Kindness, Great Compassion (大慈大悲), Great Rejoice and Great Equanimity (大喜大舍) for all beings. Here, ‘Great’ is the greatest possible.
Maha Brahmā should not be mistaken as Brahmā Sahampati (娑婆世界主), which means ‘the Brahmā who is the Sahā World’s lord’, who is the highest (i.e. most spiritually senior) Brahmā, in the highest form realm heaven (色界天) in the Fourth Dhyāna Heaven (四禅天). He was the Brahmā who invited the Buddha to teach after he attained Buddhahood. (More can be seen in the second link below.)
It is not that Brahmā Sahampati literally rules over this world, over us all, but that he is the god with the most loving-kindness, compassion, rejoice and equanimity in this world. He is thus the highest Dharma-Protector god. Thus was he naturally very enthusiastic to invite the Buddha to teach, for the welfare of all in this world.
There might be some confusion, with some seeing the Four-Faced ‘Buddha’ to represent Maha Brahmā, and some others seeing him to represent Brahmā Sahampati. Perhaps, Buddhists who are likewise confused can express respect to these gods in a general sense, taking them to represent all Dharma-Protector gods.
It should be noted that to express respect (恭敬) is different from taking refuge (皈依). While Buddhists should have respect for all good gods, especially Dharma-Protector gods, we only take refuge in the Triple Gem (三宝), which includes the Buddhas (佛陀), the Dharma (佛法) taught by the Buddhas, and the noble Saṃgha (圣僧) (which includes Great Bodhisattvas and other enlightened ones on the whole). Being the greatest source of spiritual refuge, there are no worldly (世间) and world-transcending (出世间) blessings that cannot be attained via sincere connection to the Triple Gem.
Buddhists take refuge in the Buddhas because they are spiritually greater than all humans and gods.
The Buddha’s Victory Over A God And Demon
Was the Buddha Reluctant To Teach?
Please Be Mindful Of Your Speech, Namo Amituofo!