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.
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.