Let a circle of radius \( 4 \) pass through the origin \( O \), the point \( A(-\sqrt{3}\,a, 0) \) and \( B(0, -\sqrt{2}\,b) \), where \( a \) and \( b \) are real parameters and \( ab \neq 0 \). Then the locus of the centroid of \( \triangle OAB \) is a circle of radius:
