Besides the cargo door and ball-bearing cabin floor another tech difference on the Merch was the pressurisation system, supplied only by a compressor on eng 3, rather than 2 and 3 on the Vanguard, presumably to reduce maintenance costs or fuel burn or both. So an inflight shut down of eng 3 would also require donning O2 masks.
On arrival on stand it was standard practice to shut down engines 1, 2, and 3 but leave 4 running until the cargo door had been opened by one of the pilots, presumably to reduce loads on the battery before the GPU was plugged in.