The thought of a 5th Romanov daughter is plausible because Alex and Nicholas knew that because of the blood disorder Alexei would not live long enough or have the physical ability to rule, they needed a male heir. Even today it is shocking to some people when older women have babies, how more so in Alex day it would have been socially unacceptable, she had grown daughters. That is precisely why they would have kept it a secret, that she was even expecting, or that the baby was a girl, so they didn't announce it. Let's not forget even Alexei's illness was kept a secret for years after he was born, (see newspaper clipping this website) family, servants and those close to the family would have kept it a secret, it was a different time when loyalty meant more than it does today. I do not believe the woman in Nederlands is a Romanov, and I am not saying they gave the 5th daughter away. Just plausible justification for reasoning on a 5th daughter.