I never told my children, and did not find it necessary to do so.
They and their families live in other states.
Unlike Pat I would hold off until they reach a responsible age or their schooling and outlook appears to have an outlook on gender issues of today.
If they actively talk about gender issues at age four maybe you share something with them now or at least educate them with your views.
Mmmm! Reading Pat's post again I feel that I am repeating what she said.