4:1-20
The census was now broken down further, with note being taken of those between 30 and 50 years old. They were to do work in the tent of meeting. The Kohathite priests had to be very careful in carrying out these tasks. Aaron and his sons took care of actually handling the holy items, the Kohathite priests were to ensure that they did not actually touch the holy things, otherwise they would die. The Kohathite priests were to be anointed by Aaron and his sons, but they were to not even look at the most holy things. We are not treat the things of God lightly.
4:21-28
Next we have the sons of Gershon, and once again it is the 30-50 year olds who are eligible to serve in the tent of meeting. They were to handle the curtains and the goatskin covering. Remember that the tent of meeting would need to be moved every time the camp moved. There is no severe warning for them, presumably because their task was far less likely to bring them into contact with the holy things.