The retarder together with the decompression brake (CRB) offers 1136hp of braking 4700Nm of the retarder of the G25 gearbox that you drove in the video. The old gearbox (GRS905) offered a 4100Nm retarder that was in the 410, 450... maybe in the 410 for example the exhaust butterfly brake entered in smaller stages together with the retarder, but I'm not sure. For use in Japan, just the CRB with +-476hp is enough. Mounting the retarder in a 4x2 truck is an exaggeration in European and Japanese terms in my opinion. A 6x4 truck that carries more weight, this makes more sense. In my country (Brazil) the retarder is not mandatory and is normally used in 6x4 trucks with a total weight of 74 gross tons.