I'm not sure about other lines besides carnival , but I do know Carnival will charge the full 5th passanger rate for your infant. I believe Norweign Cruise lines will only ask you for port charges. I'm going on a cruise in April with my infant and Carnival is making me pay for my infant. We're a family of 5 too! and it gets expensive esp. during holiday weeks.
Well the 3rd, 4th, and 5th passenger rates in the same room are much reduced from the 1st and 2nd person in the room rate. But yes you do have to pay more then port charges for the baby. Although I have seen Carnival run specials about once a year where the 3-5 person in a room only pays port charges. But I would imagine those are for non-peak sailing times when they are just trying to get bodies on board.
We're travelling on carnival too (january) and thought it was quite expensive for the 3rd and 4th guest. There was a special at the time we booked and with flight (from canada) and cruise it ended up being cheaper to buy 4 adult tickets than 2 adult and 3rd and 4th passenger. We ended up getting 2 cabins with connecting door.
I agree that paying for an infant is outrages esp. because they can not take part in any entertainment or food while on board. On my cruise the 3rd,4th, and 5th person rates are not much below the 1st and 2nd person rate. It's only $200.00 less. Please don't think I'm complaining I'm simply agreeing. The bottom line is I would never go without my infant so if I have to pay I have to pay. But it's always nice to save money were you can, but it won't happen when it comes to Carnival and infants.