La Grenouille is pretty much the last of its kind, i.e., NYC's Grand Dames of haute French cuisine. The only other one left is Le Perigord, and based on our experience there in October of '09, it does not compare.
So, on to other options.
In the four-star category, my first choice would be Eleven Madison Park -- no surprise to regulars here. Le Bernardin (if you love fish and seafood), Daniel, and Jean Georges are also perfect for celebrating a special occasion.
Cafe Boulud is a three-star, but Chef Gavin Kaysen's cuisine is, imo, of four-star quality. The ulterior epicure chose the meal he had there in May as his best meal of 2010. Happily, Mr. R. and I were along for the seriously delicious ride. You can see his photos of that meal here: http://www.flickr.com/photos/ulteriorepicure/sets/72157623892837723/
I think it in part depends on how strictly French the poster wants. EMP is not a French restaurant (though I love it).
missgmf, you should read the menus on line for all the places suggested and see what you think.
I really like Cafe Boulud - check out the menu. Daniel, Jean Georges, Le Bernardin and Bouley too.
20 East 76th St., New York, NY 10021
1 Central Park W, New York, NY 10023
155 W. 51st St., New York, NY 10019