In Chapter Four, how is "knowing nothing whatever about him" a "subtle tribute" paid to Gatsby by his guests?
There is some sarcasm in this statement and perhaps a greater double meaning as well.
This phrase appears after a snippet of conversation that Gatsby's party guests have insinuating that Gatsby is not who he appears to be and is, in fact, a bootlegger, a cousin of Von Hindenburg and "second cousin to the devil".
Suggesting that most of the guests did Gatsby the tribute of knowing nothing about him, Nick references 1) the idea that the party guests accept hospitality offered to them from Gatsby and 2) these guests look past the sinister gossip to do so.
Ironically, it is the party guests who espouse this sinister gossip. They are the ones painting the ugly picture of Gatsby and they feed his reputation for ostentatious parties by attending them.
So it is not in tribute that they attend Gatsby's parties and not exactly in tribute that they "look past" his reputation, as they, in a sense, are the parties and they certainly are partly responsible for his reputation.
On a basic level, Nick is saying that the party-goers are shallow hypocrites deserving of the same kind of derision they so freely pass out. Nick's statement, seen in this way, is one of sly derision in itself.
This comment from Nick is an example of satire. He is suggesting that the people who attend Gatsby's parties do not attend because they know and like Gatsby but because they want to reap the benefits of his lavish hospitality and like to gossip about how Gatsby came to be so rich. This idea was confirmed in Chapter Three when Gatsby's guests proved that they do not know anything about Gatsby at all. Some said that he graduated from Oxford University, for example, while some thought he was a spy and one person accused him of killing a man.
As such, this tribute to Gatsby is so "subtle" that it is not really a tribute at all. As Nick observes, these people are only interested in Gatsby because he is rich and successful. They have no interest in finding out who he really is or getting to know him on a personal level.