I take it you mean a 4
class config. 4 seats on a huge plane... wow think of the leg room
I didn't even think if there was still 4 class 747s flying around - my guess is they are all 3 class now, but in any case, if it were a downgrade from 4 to 3 classes, there would likely be either the same or more Business seats.
To answer your question, however, if you are downgraded, you would mainly have to deal with US Airways, since they issued the ticket. You can attempt a reroute if there is availability and ask for a waiver of the change fee on the grounds of involuntary downgrade. You can also ask for the same flight on a different day - again, subject to availability - but I would suspect that the modified ticket would still need to conform to the rules regarding stopovers and open jaws.
Qantas won't be able to do anything
directly to help you, unless you are downgraded on the day of travel. If you can't get reorganised with US DM, then you might be able to ask QF for some help to locate an alternative flight(s), of which they may then be able to modify their part of the booking. You would then have to call US DM to have the itinerary revalidated and reticketed (QF will effect something on the ticket which US will see as a "flight change", a similar signal to that if your class is downgraded in the first place).
Whatever you do, if you find your ticket is changed in any way, you should call US DM.