Preheat oven to 350ºF. Spray 13 x 9 in. pan with vegetable pan spray or Bake Easy! Non-Stick Spray.
In small bowl, combine flour, baking soda and salt. In small saucepan, melt butter and sugar with water; stir until sugar is dissolved. Add 2 cups chocolate chips; stir until melted. Remove from heat. Stir in vanilla.
In large bowl, beat eggs with electric mixer. Add chocolate mixture; mix well. Add flour mixture; stir until just combined. Stir in remaining 2 cups chocolate chips. Spread batter in prepared pan.
Bake 25-30 minutes or until toothpick inserted in center comes out almost clean. Cool completely before cutting.