SKMapAtFour

type SKMapAtFour<K extends [SK, SK, SK, SK], V extends SerializableValue> = { 
  (prefix: K[0]): MapStorage<[K[1], K[2], K[3]], V>; 
  (prefix: [K[0], K[1]]): MapStorage<[K[2], K[3]], V>; 
  (prefix: [K[0], K[1], K[2]]): MapStorage<K[3], V>; 
};