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Hopefully the people on this thread with more experience will answer this question, as I have no personal experience with either option. From a logic point of view, since the two sets of chicks are 4 days apart, I would leave the early hatched chicks with the hen, then take the late eggs to the incubator when (if) she leaves the nest to take care of the early chicks. Chicks imprint on the hen (or on who ever is taking care of them) during the first three days. If you take away the early chicks on day one and don't return them until 4-5 days later, they won't imprint on her properly. On the other hand if you leave the early chicks with her to imprint, and hatch the late chicks in the incubator, you can possibly return the late chicks to the hen in time to properly imprint. I say possibly because she may not accept more chicks after her current clutch is 4 days old, or the age difference between the early and late chicks may be such a difference that the older ones pick on the younger ones.