My answer is number 4.
The sequence of events has been described in detail

Yea, somewhat time-taking to think.

WHY I CHOSE #4 : ;

1. From the passage, we get to know that the writer had spent time on write-ups identifying with people's/human's behavior previously, but later on, he got to pay more rapt attention on the causes, the influences, both psychologically and mentally.
   I guess the inspiration came from, in the normal life setting humans are being judged by their actions, but the truth of the matter is that, there are things either good or bad a person had worked the mind on, before there could be any result. And so, the novelist decided to focus more on this things.

WHY NOT #1,#2,#3 ?

Clearly, not #1, #2, because he shifted his writeup attentio from totally just human Actions to motives.

Not #3,

Because, it says the psychological and mental complexity was dealt with in a subtle way which is not so. That his what he made very obvious in the writeups.

Thanks 😃