A flashback is an intense feeling triggered by something in the past. It is very persistent and it is never forgotten. It is quite personal.
In this passage, Conrad deliberately tells a historical tale about something the character talking has read, something that may have intensity for him and it may even be something that has governed his behavior every time he thinks of it. But that is far too cerebral to be considered a flashback.
Two is actually a pretty good choice, because Marlow has a marvelous vocabulary when he speaks of darkness. It is his driving force, and you'd best understand that when that word is used (and it is used in the title) that he is going to tell us something very important or least to him.
Four is also going to be important because he sharing something very personal. The fact that he was a fresh water sailor is not trivial. It means something to Marlow if not to those listening.
Three defines the darkness. It is black punctuated by these small flames. It is not in Marlow's experience and it is not one of his flashbacks. Three is definitely not your answer.
One is a commentary on efficiency. It is a philosophical conclusion about who Europeans are. It is not part of the flashback properties.
So it is between two and four. I would pick four merely because it is Marlow's own experience and this passage is right in Chapter one. Conrad is leading us somewhere very important and we are tuned into his past and the darkness he is about to tell us about. It's the way good novels are written.
Answer: Four.