The road bridges over the rail line are more variable and some would need to be completely rebuilt. Burke Road and Burwood Road bridges in Camberwell seem to have just enough space for four tracks, but the extra track and platform at Camberwell Station on the south side would require changes to the current train stabling layout. The trench west of the station also seems big enough, because of the Alamein line shuttle siding. The creation of the Burnley-Newport line in Stage 4 would presumably negate the need for Alamein line shuttles, so it can be removed. Power Street, Elgin Street, Burwood Road and Yarra Street bridges in Hawthorn and Swan Street bridge in Burnley only span three tracks and would all need to be rebuilt.
The additional fourth track would be added on the south side of the current line between Camberwell and Glenferrie, but would probably have to switch to the north side at Hawthorn Station. The fourth track would then switch back to the south side of the current line after Hawthorn to cross over the Yarra River. I should also note that the rail bridge over the pedestrian underpass at Domville Avenue/Wyuna Avenue/Railway Place in Hawthorn doesn't appear to accommodate four tracks and would have to be extended.
It looks the purpose of quadruplification is to reconfigure the tracks into 2 Belgrave/Lilydale tracks and 2 Alamein/Glen Waverley tracks, called the Ringwood Loop line and the Burnley-Newport line respectively in Stage 4. Presumably the Belgrave/Lilydale pair would handle express services and the Alamein/Glen Waverley pair would handle stopping all stations services, similar to the Cranbourne/Pakenham and Frankston lines between South Yarra and Caulfield.
It would be a great opportunity to fix up all the horrible trackwork at Burnley and Camberwell. It could also include the grade separation of Madden Grove, by extending the Glen Waverley down-line flyover to a bridge over the road, and bringing the up-line up to the same bridge. The only problem with this would be the train stabling on the south side of Madden Grove - could this be relocated?
I suspect it won't happen for a very long time but thanks for reading.