I have to correct myself, as I suggested the middle of the zone for the second runner is the 400m start. It is not, as was noted in a previous post a while back. It is the start of the 800m, if that event is run in lanes (usually not). Some tracks have a line indicating the middle of the 4x400 zone, but often it is a guess. When you see auto results, as I mentioned, they don't split the first runner *except perhaps in lane 1, because each lane has a different location for the middle of the handoff zone.
Sorry for the confusion. I should know better, having run this event and coached it for a long time...ha, ha.