IEC 61300-3-4:2023 is available as IEC 61300-3-4:2023 RLV which contains the International Standard and its Redline version, showing all changes of the technical content compared to the previous edition.IEC 61300-3-4: 2022 describes the various methods available to measure the attenuation of optical components. It is not, however, applicable to random mate attenuation measurements as described in IEC 61300-3-34 and IEC 61300-3-45 nor for attenuation measurements of dense wavelength division multiplexing (DWDM) devices as described in IEC 61300-3-29. This fourth edition cancels and replaces the third edition published in 2012. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: a) addition of Clause 3 containing terms, definitions and abbreviated terms; b) addition of a new LSPM measurement method, insertion method (D); c) addition of Annex A describing attenuation measurement of multicore fibre; d) changed reference test method to insertion C and alternative test method to substitution or insertion D for power meter and type 4 DUT.