Knowing not so much about Tor but some about math: the number of nodes you need to compromise in order to de-anonymize a Tor user is exponential in the number of hops. Google says there are roughly 7000 Tor nodes, including 2000 guards (entry) and 1000 exit nodes. If you have a single hop, there's roughly a 1/1000 chance that you will connect to a single malicious node that can de-anonymize you, going up linearly with the number of nodes an attacker controls. If you have 3 hops, you have a 1 in 1000 * 7000 * 2000 = roughly 14 billion chance. 2 hops would give you 1 in 2 million, 4 hops would give you 1 in 1000 * 7000 * 7000 * 2000 = 98 trillion. In practical terms 1:14B is about the same as 1:98T (i.e. both are effectively zero), but 1:2M is a lot higher.

1 = no privacy from relay

2 = risk of collusion between relays

3 = goldilocks default

4 = ... actually, you have more attack surface and you are more susceptible to fingerprinting because everybody else is using 3, so you're timings etc help identify you

So the default is 3 and nobody ought change it! Use 3 like everybody else.

The exception is .onion sites. TOR actually deliberately defaults to 6 hops when accessing .oninon sites - 3 to protect you and 3 to project the site.

because then there is at least one node that knows neither the source nor the destination of a request

The law of diminishing returns