First, congratulations on the birth of your second child, Dreamer! Second, I think it will fit, especially if the 4 mm ring has a flat or curved edge (rather than a milgrained edge). For some reason, I can wear a smaller size in flat bands, and need a larger size in milgrain-edged bands. But I do have the same question as a few others -- with your first pregnancy, did your fingers quickly return to pre-pregnancy size?