First of all, we can calculate how many 4-meter sections can be installed on this path. The path is 20 meters long, and a street lamp is installed every 4 meters, so the number of sections that can be installed is 20/4=5.
Then, we can calculate that in these five sections, each section needs to install a street lamp. So a * * * needs five street lamps.
To sum up, it takes 5 street lamps to install a street lamp every 4 meters on one side of a 20-meter-long path.