No person shall place any box, bale, package, lumber or other thing on any sidewalk by reason of which any such sidewalk shall be obstructed; provided that merchants, tradesmen and persons while engaged in receiving or forwarding goods or any other commodity may use a portion of the sidewalk in front of where the goods or other commodity is to be received or shipped, for a period not exceeding four (4) hours in any one day. At all times and in all instances of such necessary obstructions at least four (4) feet in the clear of such sidewalk so obstructed shall be left free and open to the unobstructed use of pedestrians.