so four-faced buddha chant what?
Mantra chanting can be in Chinese, Tibetan, or Sanskrit (an ancient Indian Language).
Usually, if I chant a mantra, I would just focus on the chanting... it's OK to chant it in any languages. What's most important is that you chant with mindfulness/awareness... and not be bothered & affected by your own wandering thoughts. Chanting in Buddhism can be considered a form of non-seating meditation.
Our historical Buddha is Shakyamuni Buddha, borned lived than 2,500 years ago in India. The Theravadin tradition (Thai Buddhism source is the Theravadin stream) only reveres the Shakyamuni Buddha (e.g Amitabha is not being taught in Theravadin doctrines).
The Four-Faced Buddha, or Phra Phom, is not actually Shakyamuni Buddha, but the hindu god Brahma (since the Thai culture is heavily influenced by Brahmanism).
Namo Shakyamuni Buddha
Namo Ben Shi Shi Jia Mou Ni Fo
so there are no chants for Phra Phom?
No. Phra phom is considered a samsaric being still not free from birth and death. The refuge vows of a Buddhist states that we should not take refuge in worldly deities. However in history, it was Brahma who invited Buddha to teach the dharma. So it is fine to pay reverence and respect to the Brahma.
Those who are worthy of offerings are arhants* and above.
* Worthy One; Saint, having attained cessation of sufferings and being freed from Samsara
