Intro
As we all know, Mirza Mubarak Ahmad was the youngest son of MGA.  He died abruptly in 1907 and this death sent shock-waves through MGA and his followers, since MGA was always predicting the death of his opponents, thus this death, coupled with the death of Abdul Kareem made Ahmadis doubt the truthfulness of MGA.

Some Quotes that show that MGA added Mirza Mubarak Ahmad to the prophecy of Musleh Maud.  

Anjam-e-Athim, pg. 14, appendix, (1897), also see, Mumtaz Ahmad Faruqi, Truth Triumphs, page 31.

…then there is another ilham which is proclaimed on 20th February, 1886, and that was that God will increase Three into Four.  At that time there was no sign of my three sons (mirza Mahmud Ahmad, Mirza Bashir Ahmad and Mirza Sharif Ahmad) who are present now ; so it seemed that the ilham meant that first of all three sons will be and then another one will be added who will make the number four.  So a greater part of this Ilham is fulfilled as God has given me three sons from this marriage (the second wife) who are here ; and the next one is awaited who will increase the number into four”

A few years later, MGA wrote

Tiryaq-e-Qulub (written in 1899, published in 1902), also see Mumtaz Ahmad Faruqi, Truth Triumphs, page 31.

“this prophecy of ‘increasing three into four’ was announced on 20th February, 1886, and after the birth of the three sons e.g., Mahmud, Bashir and Sharif, it was again mentioned in the Anjam-e-Athim and its Appedix, that as informed by God, the Proomised Musleh (or reformer) who will increase three into four will now appear….”

More evidence is posted in Tiryaq al-Qulub, page 40
Under the 25th sign, MGA writes that he considers the birth of Mirza Mubarak Ahmad as a divine promise.

Mirza Mubarak Ahmad died in 1907
MGA then received additional ilhams to the effect that his God would give him another son.