I think you should be able to pick them up confidently but with due respect (i.e. don't just reach in and snatch).
Grabbing behind the neck should only be used if absolutely necessary (e.g. if you have exhausted ALL other options of getting your snake back into it's enclosure).
Reaching over any animals head is threatening behavior, some snakes are more tolerant of this, but don't expect it.
IMO only the most placid of python should be given to a 4yo to hold and care should be taken to disinfect their hands afterward to remove the risk of zoonosis. Anything above the size of a full grown mac should not be handed to the child to hold at all (again very much IMO, I know opinions vary)
