I assume that the machines can be heard from outside 
where the backup switches are. If so: 
 
Lets label the machines as ABCD. 
The brother turns of A&B before going outside. 
Outside, he flips switches in pairs till he 
hears complete silence. (i.e. none of the machines 
run). At this point, outside there will two 
switches that are off and two that are ON. 
The ones that are OFF correspond to machines 
C & D. So lets label them CD1 and CD2. Similarly 
the other two are labeled AB1 and AB2. Now 
the brother sets CD1 and AB1 two ON and turns 
off CD2 and AB2. He then walks back in. 
He will see one machine running. If C is 
running then CD1 = C & CD2 = D. If D is 
running then CD1 = D & CD2 = C. Thus 
he determines which switch is for C and 
which switch is for D. Now for A&B, he turns 
on machine A's switch inside. If machine A 
runs, then AB1=A & AB2=B. If not, AB1=B 
& AB2 = A.