While I am not from New York I am a major advocate of properly restraining kids even when it is not convenient so I did a little research and this is what I found.
"On Sunday, March 27, 2005 New York State joined more than two-dozen other states requiring that children from ages 4 to 6 sit in booster seats when riding in a car."
In my opinion, it would be better even if you only bought one of those little $13 booster seats from Walmart for travelling purposes than taking the chance. You love this little person too much to take a chance, whether it is the law or not studies have shown that kids need to be in booster seats. I know that there have been many times as we are juggling luggage and seats that I would just love to toss the things in the nearest garbage bin but when my blue eyed blonde wraps her little arms around me, it is worth all the inconvenience in the world to know that I am doing my best to keep her safe.