Since you already decided on a plan this probably will not matter. You can also go TPAC on Malaysian airlines to Kualalumpur and there are direct flights to MAA from there. Also the train from BLR to MAA takes lot more than 4 hours. Dont know about it now but 15 yrs back I remember it took almost 12 hours. If you fly to BOM, you get in around 11PM and typically you have 3AM flights to other major cities and that 4 hours is a good time to get out of immigration/customs and getting to the domestic airport terminal.