First, OM stands for optical multi-mode. Both OM3 an OM4 are 50/125 core fiber but they have different internal construction that allows the OM4 fiber to provide the same performance as OM3 but for longer distances.The reason for this is the difference in bandwidth, OM3 has 2500 megahertz bandwidth, OM4 has 4700 megahertz bandwidth.hat this translates to is longer transmission distances for the OM4 fiber. So you will need to know the distance of the fiber run in order to help decide which kind of fiber and fiber accessories need to be used.