Hollow core golf ball having a hardness gradient

A golf ball comprising a hollow core, the golf ball comprising:

• a spherical inner core shell layer formed from a stiff thermoplastic composition, the shell layer having an outer surface, an inner surface, and an inner diameter to define a hollow center;
• an outer core layer disposed about the shell layer, the outer core layer comprising a thermoplastic material;
• an inner cover layer disposed about the outer core layer, the inner cover comprising an ionomeric material and having a first hardness;
• and
• an outer cover layer disposed about the inner cover layer, the outer cover comprising a polyurea or a polyurethane and having a second hardness different from the first;
• wherein the hollow center has a diameter of about 0.15 to 1.1 inches, the thermoplastic inner core shell layer has a hardness of about 85 Shore C or greater, and the stiff thermoplastic composition has a melt volume flow rate of about 5.0 or less cm3/10 min at 230° C./2.16 kg.