Manas, Buddhi and Ahamkara are the three different sides of a triangle,' which is called Chitta. The Chitta is not a fourth, but the sum of the three: Manas, Buddhi and Ahamkara.
This is the old idea of a trinity in unity. Over and over again H. P. Blavatsky uses this summation as a fourth to her triplets, for she follows the old methods. The fourth, which sums up the three but is not other than they, makes a unity out of their apparent diversity.
Let us apply that to Antahkarana.
Take cognition. Though in cognition the aspect of the Self is predominant, it cannot exist absolutely alone, The whole Self is there in every act of cognition.
Similarly with the other two. One cannot exist separate from the others. Where there is cognition the other two are present, though subordinate to it. The activity is there, the will is there.
Let us think of cognition as pure as it can be, turned on itself, reflected in itself, and we have Buddhi, the pure reason, the very essence of cognition; this in the universe is represented by Vishnu, the sustaining wisdom of the universe.
Now let us think of cognition looking outwards, and as reflecting itself in activity, its brother quality, and we have a mixture of cognition and activity which is called Manas, the active mind; cognition reflected in activity is Manas in man or Brahma, the creative mind, in the universe.
When cognition similarly reflects itself in will, then it becomes Ahamkara, the "I am I" in man, represented by Mahadeva in the universe.
Thus we have found within the limits of this cognition a triple division, making up the internal organ or Antahkarana - Manas, plus Buddhi, plus Ahamkara - and we can find no fourth. What is then Chitta?
It is the summation of the three, the three taken together, the totality of the three. Because of the old way of counting these things, you get this division of Antahkarana into four.
[Owen Jones has a webite called 'The history and Philosophy of Hindu Yga'; it can be viewed here: ://yoga.the-real-way ]