Whiff punishment is punishing moves that hit the air (hit nothing) that are poorly spaced/positioned. It can be done via strike or throw, depending on distance and how much recovery a move generally has.
In @Force_of_Nature example of 3H+K~44, he is implying that with good application of 3H+K~44, if the opponent tries to hit a button to challenge the spacing, a 4K can whiff punish the opponent's whiffed move.
4K (sit down stun) > 4P+K (launch) > Juggle as your example of whiff punishing with 4K.
In @Force_of_Nature example of 3H+K~44, he is implying that with good application of 3H+K~44, if the opponent tries to hit a button to challenge the spacing, a 4K can whiff punish the opponent's whiffed move.
4K (sit down stun) > 4P+K (launch) > Juggle as your example of whiff punishing with 4K.