Section 4.  Every home rule charter or plan of government existing or adopted when this constitution is adopted shall remain in effect and may be amended, modified, or repealed as provided therein.  Except as inconsistent with this constitution, each local governmental subdivision which has adopted such a home rule charter or plan of government shall retain the powers, functions, and duties in effect when this constitution is adopted.  If its charter permits, each of them also shall have the right to powers and functions granted to other local governmental subdivisions.