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. G. Blavatsky uses this summation as a fourth to her triplets, for she follows the old methods. The fourth, which sums in the three but is not other than they, makes an unity out of their apparent diversity.
Let us apply that to Antahkarana.
Take knowledge. Though in cognition the part of the Self is predominant, it wouldn't exist absolutely alone, Improving your general health Self partnerships in every act of cognition.
Similarly the new other a pair of. One cannot exist separate of your others. In which there is cognition the other two are present, though subordinate on it. The activity is there, the will is there.
Let us think of cognition as pure since it can be, stimulated itself, reflected in itself, and possess Buddhi, the pure reason, the very essence of cognition; this in the universe is presented by Vishnu, the sustaining wisdom of the universe.
Now allow us to think of cognition looking outwards, and as reflecting itself in activity, its brother quality, and have an array of cognition and activity which is known as Manas, the active mind; cognition reflected in activity is Manas in man or Brahma, the creative mind, on the universe.
When cognition similarly reflects itself in will, then it becomes Ahamkara, the 'I am I' in man, represented by Mahadeva associated with universe.
Thus we have found in limits with the cognition a triple division, making along the internal organ or Antahkarana - Manas, plus Buddhi, plus Ahamkara - as well as can find no next. What is then Chitta?
It could be the summation of your 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 as well as Philosophy of Hindu Yga'; it can be seen here: http://yoga.the-real-way.com]