Let the width be x meters.
The length will thus be 2x + 4 meters.
Since the perimeter is 104 meters, we have:
Perimeter = length + length + width + width
104 = 2x + 4 + 2x + 4 + x + x
104 = 6x + 8
6x = 104 - 8
6x = 96
x = 96 / 6
x = 16 meters
Width = x = 16 meters
Length = 2x + 4 = 2(16) + 4 = 36 meters