Mass Increase: Opposing characters within 4 squares of Star Boy can't be given actions to use Charge, Running Shot, or Hypersonic Speed and halve their speed values when given a move action.

If an opposing character is 5 or more squares away from Star Boy and uses Hypersonic Speed, will it have to stop if it moves within 4 squares of Harry?
No. The description of Mass Increases specifically states that an opposing character can't be given an action to use certain powers if it is within 4 squares. In the situation described here, the action was given when the character was more than 4 squares away... and that is all the trait cares about: giving actions. Once the action has already been given, the opposing character can move as close to Star Boy as the controller likes.