Legend of Korra

The name and release date for Book 4 haven't been announced yet.
I believe Iroh is considered the leader of the White Lotus, although he is in the Spirit World.

I found two interesting quotes from the past of the voice actors behind Korra and Asami.

 Spoiler

Janet Varney - We just recorded something yesterday that’s for an episode way down the line with Eva Marie Saint [who voices the character Katara in the series], and I felt like she was talking to me, Janet Varney. There is stuff she says in that script that I was like, “Oh my gosh, I gotta remember this. This is so true. This is how life is. Life is really hard.” (Source)

From what I have read, this episode will be in Book 4. Interpret it as you wish.

Seychelle Gabriel -

This can be interpreted in different ways, if you know what I am talking about.

I am scared for the future with that quote. The season finale can't come soon enough.

There's a leader of the White Lotus but he is of no importance in the show. He's only in the first episode and a cameo in the second season. It's strange though. I would've thought the Avatar would have been the leader of the White Lotus.

That doesn't sound promising, I would have liked if the White and Red Lotus clashed head to head.