Don't have personal experience to back this up, but my guess would be that the Master's does not matter assuming you finish the PhD.
However, it would be very valuable if, for whatever reason, you end up dropping the PhD. Especially if getting the Master's is only a minor amount of extra work, I would definitely add it.
You never know what could happen over the next 4 years. You will greatly regret not getting the Master's if something unforeseen happens and you drop the PhD with nothing to show for it!