We present Karl G. Jansky Very Large Array observations of the CO(2-1) emission in the z = 4.05 submillimeter galaxy (SMG) GN20. These high-resolution data allow us to image the molecular gas at 1.3 kpc resolution just 1.6 Gyr after the big bang. The data reveal a clumpy, extended gas reservoir, 14 + or - 4 kpc in diameter, in unprecedented detail. A dynamical analysis shows that the data are consistent with a rotating disk of total dynamical mass 5.4 + or - 2.4 x 10 super(11) M sub(middot in circle). We use this dynamical mass estimate to constrain the CO-to-H sub(2) mass conversion factor ( alpha sub(CO)), finding alpha sub(CO) = 1.1 + or - 0.6 M sub(middot in circle)(K km s super(-1) pc super(2)) super(-1). We identify five distinct molecular gas clumps in the disk of GN20 with masses a few percent of the total gas mass, brightness temperatures of 16-31K, and surface densities of >3200-4500 x ( alpha CO/0.8) M sub(middot in circle) pc super(-2). Virial mass estimates indicate they could be self-gravitating, and we constrain their CO-to-H sub(2) mass conversion factor to be <0.2-0.7 M sub(middot in circle)(K km s super(-1) pc super(2)) super(-1). A multiwavelength comparison demonstrates that the molecular gas is concentrated in a region of the galaxy that is heavily obscured in the rest-frame UV/optical. We investigate the spatially resolved gas excitation and find that the CO(6-5)/CO(2-1) ratio is constant with radius, consistent with star formation occurring over a large portion of the disk. We discuss the implications of our results in the context of different fueling scenarios for SMGs.