The main features of the Government of India Act, 1919 were as follows:

1. Introduction of separate electorates for Muslims: This provision allowed the Muslim community to elect their own representatives separately. It was aimed at addressing the demands and interests of the Muslim community.

2. Devolution of legislative authority by the Centre to the Provinces: The Act granted more autonomy to the provinces by transferring some legislative powers from the central government to the provincial governments. This was done to promote self-governance and accommodate the demands for more regional autonomy.

3. Expansion and reconstitution of Central and Provincial Legislatures: The Act expanded and reconstituted both the Central and Provincial Legislatures. This was done to include more representatives from various sections of society and regions. The Act introduced the principle of direct election and increased the number of elected representatives.

Therefore, the correct answer to the question is option 4, as it includes the main features of both separate electorates for Muslims and the devolution of legislative authority to the provinces, as well as the expansion and reconstitution of both Central and Provincial Legislatures.