I will be attending a panel interview (with two men and two women). I don't know their names. I want to be more polite, but I am not sure whether it is alright to say
Good afternoon, sirs and madams
in the panel interview.
At an interview, you should not be too effusive with your greeting, or too verbose (unless invited by a leading question intended to draw you out). The interview panel makes the moves, so I suggest you be polite and uncontroversial.
Good morning / afternoon
is sufficient, with a brief look around the interviewers to make it clear you are greeting them all, and wait for them to take the lead.
When politely greeting one person, we can say "good morning/afternoon/evening", and possibly add "sir" for a man, or "madam" for a woman, although these are now very old-fashioned in Western countries, except for e.g. royalty, judges in court, etc. "Sir" and "madam" do not have plurals. To greet a group, mixed in gender, we can say "Good morning/afternoon/evening, ladies and gentlemen." ("ladies" always comes first).
Yes, but "Ladies and Gentlemen" is more conventional nowadays.
No, don't say "sirs and madams" under any circumstances. "Madams" are women who run brothels.
I would recommend any of these.
Which is best depends on which country you're in.
In the UK I have very frequently and over many years been in meetings that begin with somebody addressing the group relatively formally. If they happened all to be men, no problem: "Gentlemen" does the trick. If all were women:"Ladies" is perfect . Likewise if there were several men and women "Ladies and Gentlemen" would be quite correct. But what do you say if there is only one woman? Far too frequently I have heard some kind of embarrassed "Er, Lady... and Gentlemen", on the false basis that the singular of "Ladies" is "Lady".
The correct form has to be "Madam, and Gentlemen". If she were there on her own (and recall that we are being formal here) you would address her as "Madam". That is not changed by the presence alongside her of men. The plural of "Madam" is "Ladies".