According to one Tolkien quote from his letters at least it should. He was by no means of the opinion a child is its fathers alone. Luthien's bloodline is her bloodline. And a mother is not just a container for the child, neither genetically, socially or mytholgically (see Miriel) - a child is still more a creation of the mother (even if judically it counts as its father's in ME).
Prengnacy is a complex interaction between the organisms of the mother and the developing child, I am of the opinion Mithrellas would simply be not able to support such a fast but superficial pregnancy like a human one is. And risk this twice. It would damage her psycologically and spiritually too much. I think her body would completelly confused how to even start such a process.
So I do think 4-6 years is the lowest we can go for her two children.
@WillChan
I do not think choosing mortallty changes the biology of a given person. If it would, then Luthien and Arwen should start aging at a human rate the moment they change over, with sickness and decay included, and as far we know that does not happen. That would mean also getting menopausal, and as both of them were several thousands of years old at the moment, they would be not able to bear any children at all if their bodies simply would change instantly into a mortal woman's one (actually they both should die the very instant such a change would happen) or what - would they simple get an mortal life period of ca. 80-90 years added to their elvish part of life (if so, this should actually include new human gestation, childhood and puberty for their bodies to be and act wholly human, and that for sure does NOT happen)? This also would not work for Arwen, as she has outlived Aragorn and he reigned for 120 years. Aso elwish women seem not to have cicles, as they conceive their children at will, so has Arwen started menstruating the moment she "became human"? How so, on which basis - when would her body have the time to learn that (a human girls' body prepares for this several years long of trial and error of puberty)?- not believable.