Why do you think McMahon added the fourth assurance:

I have received your letter...with much pleasure and your expression of friendliness and sincerity have given me the greatest satisfaction...

As for those regions lying within those frontiers wherein Great Britain is free to act without detriment to the interests of her ally, France, I am empowered in the name of the Government of Great Britain to give the following assurances and make the following assurances and make the following reply to your letter:
(1) ...Great Britain is prepared to recognise and support the independence of the Arabs in all the regions within the limits demanded by the Sherif of Mecca.
(2) Great Britain will guarantee the Holy Places against all external aggression and will recognise their inviolability.
(3) When the situation admits, Great Britain will give to the Arabs her advice and will assist them to establish what may appear to be the most suitable forms of government in those various territories.
(4) On the other hand, it is understood that the Arabs have decided to seek the advice and guidance of Great Britain only, and that such European advisers and officials as may be required for the formation of a sound form of administration will be British.
(5) With regard to the vilayets of Baghdad and Basra, the Arabs will recognise that the established position and interests of Great Britain necessitate special administrative arrangements in order to secure these territories from foreign aggression to promote the welfare of the local populations and to safeguard our mutual economic interests.

I am convinced that this declaration will assure you beyond all possible doubt of the sympathy of Great Britain towards the aspirations of her friends the Arabs and will result in a firm and lasting alliance, the immediate results of which will be the expulsion of the Turks from the Arab countries and the freeing of the Arab peoples from the Turkish yoke, which for so many years has pressed heavily upon them...

It was with very great relief and satisfaction that I heard of the safe arrival of the Holy Carpet and the accompanying offerings which, thanks to the clearness of your directions and the excellence of your arrangements, were landed without trouble or mishap in spite of the dangers and difficulties occasioned by the present sad war. May God soon bring a lasting peace and freedom of all peoples.

I am sending this letter by the hand of your trusted and excellent messenger, Sheikh Mohammed bin Arif bin Uraifan, and he will inform you of the various matters of interest, but of less vital importance, which I have not mentioned in this letter.

(Compliments).
(Signed): A. HENRY MCMAHON.