The wizard has answered similar questions on other Blackjack games where, essentially, similar anomalies occur - if you stuck to Basic Strategy why say what to do on places you can't get to.
The answer is that it caters for the situation where you've gone totally mad (or misread your cards) and now find yourself in a strange situation and saves the type of question well what do I do with this now. Sometimes it is better to state the obvious - for instance there is one game where having split Aces you would Double Down 21 - so in other games it's best to confirm you stand.
Meanwhile note that most of the time you would have split the 2's so couldn't have doubled the 4 to get to a three-card total of 6; and perhaps there should be a indication of what to do with 4 when you can't split anymore.
Thanks, Charlie, that makes sense.