Combine 4 cups flour, baking soda, and salt. In a separate bowl, beat butter, granulated sugar, and brown sugar until creamy. Add eggs and vanilla, mixing well. Add flour mixture and mix well.
Divide dough in half. To one half, add cocoa and mix. To the other half, add 1/4 cup flour and mix. Stir into each dough 1 cup oats and 1 cup chocolate chips. Cover and refrigerate for 2 hours.
Preheat oven to 375°. Combine 1 tablespoon of each dough, twisting together to make one cookie. Place 2 inches apart on ungreased baking sheets. If dough becomes too warm, refrigerate for a few minutes then resume forming cookies.
Bake about 10 minutes or until lighter dough is golden brown. Cool on sheets for 2 minutes. Then, remove cookies to wire racks to cool completely.
Makes about 4 dozen large cookies.