Well, 'The Lost Radiant' is quite popular. It tells a story about a character who believes they might be a lost Radiant but has to prove it to themselves and others in a world that is skeptical. 'Shardblade Origins' is another. It tries to create a backstory for how the Shardblades came to be in a more detailed and unique way compared to what's in the official books. And then there's 'Lift's Secret Missions' which is all about Lift going on some covert operations that are not part of the main story but are really exciting.
I'm not sure specifically which four novels are being referred to as 'secret' by Brandon Sanderson. He has a large body of work, and perhaps these are upcoming projects not yet fully revealed.
Dalinar is another important character. A highprince, he struggles with his past actions and his visions. His path towards redemption and his role in the events of the Stormlight Archive short stories are crucial as he tries to lead his people in the face of impending doom.
The word count in a Brandon Sanderson novel can differ greatly. On average, they tend to be in the range of 120,000 to 150,000 words. But some of his epic fantasies might exceed that.
Brandon Sanderson usually starts with a core concept or a unique magic system and builds the plot around it. He often plans out the major plot points in advance but leaves room for flexibility and character-driven developments.
Brandon Sanderson has written a substantial number of novels. His works include popular series like The Stormlight Archive and Mistborn. To be precise, it's a constantly growing number as he continues to create new stories.