Octagons seem to be "the shape of the year". Sue has answered similar questions twice in the last few days. I am not quite sure what you mean by "4 ft-5 ft around the tree" so I have a diagram to try to help answer your question.
The relationship between w and s is
s = (√2 - 1) w = 0.414 w
You can see how to arrive at this relationship by examining Sue's answer to a similar question.
If w is 5 feet for the octagon you are constructing them
s = 0.414 5 = 2.07 feet
so cut eight pieces 2 feet long at an angle of 112.5o and they will fit together to form a hexagon.