Four rail track - its correct name is gauntlet track where the rails of two gauges are laid within each other as with the Eastern most track over Dudley St which is the States only current piece of gauntlet track . Forget this in mainline use over any distance because of the cost of extra wide special sleepers, special turnouts and the necessity to have wider earthworks, formations and most importantly wider bridges (piers and beams and decking).
Think outside the square ; south of Seymour a very workable solution would be to use the existing Up bg line basically converted to sg, but using bits of it dual gauged as necessary and connected to the remaining bg line to provide bg crossing loops as necessary for the remaining bg passenger services . Dual gauge all the way would not be required for the volume of bg traffic, and dual gauge track is much more expensive to maintain .