In common usage, the essence of something is its inherent quality or central, defining feature.
In 4th Way discourse, a man's essence is the totality of the qualities or propensities he is born with. As opposed to this, personality is the totality of the acquired or learned patterns of thought and emotion.
Essence is not directly observable since all interaction with people normally passes through the filter of personality.
Modern psychology attributes about 80 percent of observable characteristics of human psyche to factors of nature and only 20 percent to factors acquired from environment. Physical or psychological reflections of the 4th Way idea of essence would include things like features of the endocrine system having to do with stress and flight/fight responses or inherent abilities such as memory or IQ or predispositions such as verbal vs. visual thinking or left vs. right brain dominance.
There is however more to the concept of essence than this. Essence includes the concept of fundamental character, as in whether one's center of gravity is in the moving, feeling or thinking center. Even further, essence would include an idea of invariable moral character, i.e. tending towards truth or lies or service to self or service to others. There is some evidence that these features are genetically based and also reflected in observable brain functioning. For example, psychopaths, which would represent a serious flaw of essence, have distinctive brain electric patterns.
We could say that essence is what is left when the conditioning of culture and the multiple faces and acquired roles of little I's are stripped away. The Work may in principle bring essence and personality closer to each other. Gurdjieff comments that the education system of his time, and the modern one with all the more reason, favor personality at the cost of essence development by from the get go teaching hypocrisy and by teaching children to distrust their own senses. Personality is the only part that can thrive in a context of culturally conditioned lies and arbitrary value judgements.
Primitive societies are more favorable for essence development and make individuals that are less contradictory and maybe healthier in essence terms. However, they are no better for esoteric development because their cognitive or 'personality' side is under-trained.
Any work on essence needs to proceed through personality, since personality is the one part of man's inner life that can be reached. Disciplines of personality, such as self-remembering may in time benefit essence, although the 4th Way teaching is generally vague about any work on essence. Gurdjieff says that most often the growth of essence stops around the age of seven and man's essence is usually left in a state of stupidity or weakness.
There exists a set of psychophysiological features of man which are life-long or very slowly changing and which correspond to Gurdjieff's description of essence. The 4th Way and psychology seem to have reached compatible conclusions in this area.