So, we asked this question to Nokia execs during the Q&A session and as per the reply we got, it seems Lumia 638 will be the first 4G / LTE Lumia device to be launched in India.
This is quite possible given the fact that Lumia 638 is the China Mobile variant of Lumia 635 with appropriate TDD-LTE bands suitable for India. It has support for Band 40, which is the prevalent LTE band used by operators in India as of now.
LTE network band: 3, 7, 38, 39, 40, 41
You would be happy to know that Lumia 638 comes with 1 GB of RAM and hence is an upgrade over Lumia 635 in itself.