I'm pretty sure you would only want two crossbars.
If you look at the diagram in the following link you can mouse over the different parts:
http://www.courtesyparts.com/pathfinder ... _6956.html
I'm as confused as you are, however, because it looks like the crossbars alone isn't enough.
It seems you need something like 8 of these:
http://www.courtesyparts.com/73850aa-bo ... 63203.html
And 8 more of these:
http://www.courtesyparts.com/73850a-bol ... 63202.html
Along with 2 of these:
http://www.courtesyparts.com/73834n-sta ... 63200.html
And 2 more of these:
http://www.courtesyparts.com/73835n-sta ... 63201.html
Plus the crossbars of course x2:
http://www.courtesyparts.com/73807n-cro ... 23366.html
You might just want to email them and ask, because I'm just guessing at the above. Maybe if I had an engineering degree instead of history it would help

. It gets pretty costly to order all the above, so I may just go check out a local junk yard first.
Anyone who has done this care to share what they had to order?
Thanks!
Jason
Edit: Whoops too slow! Thanks for the answer.