I believe what you want is the include directive. For example in 1.rst you will put
####
Part
####
*******
Chapter
*******
Section
=======
.. include:: 2.rst
.. include:: 3.rst
.. include:: 4.rst
This will include 2.rst and however many additional files you want in the current file and continue the current structure layout. Also, change your index.rst to be just the 1.rst.
.. toctree::
:maxdepth: 3
1.rst
part2.rst
part3.rst