Classes defined in PostPolictyV4.java are not as perfect as they can be.
Enhancements and bugs are listed below:

class PostPolicyV4

• the class description is stingy, need to put more words and point to usage example
• public methods miss Javadoc
• no verification for given URL string is a valid URL
• BUG: getFields() returns the map as is, so one can modify PostPolicyV4 objects after creation
  (suggested fix: make fields immutable)

class PostPolicyV4.PostFieldsV4

• BUG: public method getFieldsMap() returns map that allows to alter PostFieldsV4 instances without Builder (no control)
  (suggested fix: deprecate method getFieldsMap(), make the map immutable, define getters)

class PostPolicyV4.PostConditionsV4

• BUG getConditions() returns mutable Set
• BUG getConditions() exposes non public class PostPolicyV4.ConditionV4
  (suggested fix: make PostPolicyV4.ConditionV4 public, make conditions immutable)