the two in back definately look like Mille Fleur......the other two yellow chicks i think might be cochin. If you can take pictures of the wings extended we could tell you for sure. i'm only basing the cochin off the fact the yellow one on the left looks bigger then the Mille's and their wings look like they might be all white. If you know what hatchery they used it would be easy to narrow down. Personality wise D'uccle are usually very friendly, easy to handle, not flightly but some dont like when you go to pick them up but once you have them settled in your hand they calm. Mine seem to do fine in the cold of northern michigan and the heat so they are hardy. Hatchery birds can be less hardy though, so they are not always a good example of the breed over all.
EDIT: the more i see this pic the more i am wondering you the yellow fluffs have all white wings or white and gold aka golden neck