It all depends on what you're looking for. If you're just looking to treat yourself you can go to Sak's or Bloomie's and find just about anything. In SoHo you can also find Prada, Marc Jacobs, Chanel, Victoria's Secret, Dolce & Gabbana, Urban Outfitters, J. Crew and Calvin Klein.
You'll probably burn through it quick, but if you have four days, maybe you'll want to shop around for the first day or two before making purchases.