It depends on how feathered out they are and if they have ready access to the lamp during the day when they need it. The rule of thumb is that they start with needing it around 95 degrees when they hatch, then knocking it down by 5 degrees a week. That would put their needs around 75 degrees at 4 weeks. I did put my chicks out at 4 weeks, but they were fully feathered and it was around 70-85 out, they also had a lamp with them at night.