Does the K4 support KF8 yet? If so @sriramvenkatesan you can convert the TXT to KF8 with the Tamil Fonts embedded.

Or if you don't mind reading PDFs on the K4, just open the TXT file in MS Word (or some other editor), apply the Tamil font to all of the the text, and save it as a PDF.

EDIT: If you create a PDF, I would make the document size approximately 3.4" W x 4.8" H. Otherwise the font size might be difficult to read on a Kindle. Also use a very small margin--no more than 0.1" or there will b a lot of wasted space.