To find the ratio of the length of a regular pentagon's side to that of a regular hexagon, you can simply divide the length of the pentagon's side by the length of the hexagon's side.
Ratio = Length of Pentagon's Side / Length of Hexagon's Side
Ratio = 16 mm / 4 mm
Ratio = 4
So, the ratio of the length of a regular pentagon's side to that of a regular hexagon is 4:1.