Look at it this way. What would you say to yourself if there were an accident caused by there not being enough support?
For 4 posts, the additional cost would be nominal, but, even if it doubled the cost, there is no price on the safety of my family.
Here is some info from decks.com:
It is not that expensive to upgrade to 6”x6” and the extra material allows for stronger connections at the top of the support to hold the beam in place. Taller wood supports have a tendency to bow or twist after the drying process if they are of the smaller sizes. We have seen a 4”x4” support posts bow as much as 6” out, causing the deck to sag 1”. If this has happened to your deck, the supports should be changed. The larger sizes tend to be more stable during the drying process.
Also, here is the link for some info on what heights are permitted for which size posts:
http://www.decks.com/articles.aspx?articleid=29
Good Luck, Dan