Prepare baking pan by lining pan with heavy tin foil or parchment paper, then lightly grease and dust 9x13 pan with flour on sides and bottom of pan. Knock out any access flour and dispose of. Can also use cooking spray.
Cut the butter into 1 inch slices and place butter and cream cheese in a heavy saucepan and cook over very low heat, whisking occasionally until melted. Turn heat off and leave saucepan on burner.
Grate zest from 4 lemons and place sugar and lemon zest into food processor until zest has broken down, about 10 seconds.
Using a large mixing bowl, add eggs and lightly beat, then add in lemon/sugar mixture and lemon juice just until incorporated. Add to butter/cream cheese mixture and blend together.
In a small bowl, whisk the flour, salt baking powder and baking soda together to combine.
Sift dry ingredient mixture into the large mixing bowl with the wet mixture, then whisk mixture until just combined. (Batter will be lumpy)
Pour batter into prepared pan and spread evenly.
Bake 32 minutes, until a toothpick inserted in the center comes out clean and top is lightly browned.
Transfer pan to cooling rack.