What is meaning of +- 4D in eye for job application requirement

power 4dAFL's OPM5 and OPM4 Optical Power Meters for accurate fiber optic testing. Featuring Wave ID, rugged desn, and compatibility with various networks. ... -4D (Filtered-InGaAs) for hhDemension : 23.5 x 17.5 x 5.2 cm. Output Power in 4 ohm : 1 x 1500 W. Output Power in 2 ohm : 1 x 3000 W. Output Power in 1 ohm : 1 x 4500 W.