It's 4, not 3

Well, for the first time ever (in over a year of frequent air travel), I had the bad luck to get the picky security lady.  Oh, you know the one - she reads the size label on each of your toiletries and delights in telling you which ones are not allowed "That's a four ounce container, nothing over three ounces is allowed".  Of course, reason is of no help "But, if it's not full, then it's not actually four ounces is it?"  So, if I look a bit scruffy at this week's events, you'll know why.