With Australia still having 10 wickets in hand and enough time to win, this is the situation where I hope Cummins considers promoting Marsh to #3 if either Khawaja or Warner gets out.
From doing this, Cummins should allow Marsh to bat at #3 and play his natural game: bat the way he did in the first innings at Headingley (like Gilchrist did at the WACA in 2006 vs England), hopefully smacking at least a few boundaries at the minimum. A quickfire 30-50 runs or so from him would get the Poms nervous, sending a strong message to them that Australia is there to win the match. Put some pressure on the Bazballers, rather than bat the way they did in the 1st session on day 3. Batting Geoff Boycott-style would work if they don't lose wickets. However, once Khawaja and Labuschagne got out in the first innings, the other batsmen panicked under pressure and began losing the remaining wickets in regular intervals.
By implementing this strategy of Marsh at #3, it allows Australia to go for the win, whilst also giving them the option of shutting up shop and batting for the draw if a cluster of wickets fall and they don't believe they can chase the target of 384. However, if Australia is planning to do the exact opposite and block England right from the start of day 5, it creates that nervous tension on the pitch that then filters down to the other batsmen in the dressing room.
If the plan works, it works. If it doesn't work and Marsh gets out cheaply, all is not lost: there's still Labuschagne/Smith/Head in the batting lineup to come.
That'd be the advantage of having Marsh come in with 9 wickets in hand vs. arriving at the crease in his usual #6 position with 6 wickets in hand, and potentially Labuschagne/Smith/Head already back in the pavilion by then. In that scenario, Marsh would be required to play more conservatively (like he did at Old Trafford) if there's still 100+ runs required for victory.
