To Treat Hyperpigmentation, Do I Apply Hydroquinone and Tretinoin to the Entire Face or Specific Parts?

I have PIH and hyperpigmentation from acne. My doctor prescribed .05% Tretinoin cream and 4% Hydroquinone. Do I mix both creams and apply it only to the hyperpigmented areas or the entire face? And if I apply it only to the hyperpigmented areas, do I apply tretinoin to the rest of the face? And how long does it take to typically see the effects? I have tan Asian skin. Based off online, maybe not entirely reputable sources, I have type IV skin.