In the beginning, Mohinder has an Indian accent. But then, by Episode 4, it's British with a tinge of Indian. Confusing? Definitely.
Did anyone else notice this?

Yeah, when I was rewatching the first episode, I noticed that his accent sounded different too. I guess they decided that Mohinder would sound better with a British accent.

I noticed this as well, and its never made sense to me. If he was born and raised in India, then why would he have a British accent?

Because the UK occupied India for a good while. India's English is British, not American.

actually Sendhil said in some interview that someone suggested him after a few episodes that he should have British accent...or the other one, I don't rememebr..anyway, he said, that he had to tone it down a little more in each next episode, and pass seamlessly to the other accent in next episodes.

Other than them being a former colony of the Briton, the "Subsidiary Official Language" of India is English, right after Hindi. The reason for this, other than being a former colony, is there are hundreds of spoken languages on the subcontinent. English is a simple unifying language that many Indians speak already. That being said, many Indians live and learn in the England to this day.

I guess everyone knew this stuff except me.

Well, knowing is half the battle right? So now you are 50% more prepared!

LOL, never really notice the change, you guys are very perceptive...