I have only seen flatpack transistors being used for this purpose where
we're talking big power levels, such as in PA amps.

However, repairing this Chinese crap every day of my working life, I can
say
that it is extremely common to find one or more flatpack devices,
additional
to those being used for the output stage, mounted on the same heatsink. I
guess this is to cut down on cost. Invariably, these 'extra' devices, are
simple linear voltage regulator pass transistors, and that is what I would
be fairly sure that the OP's fifth transistor would be.

There's also the fact that there's only one extra transistor. Wouldn't a
two-channel output stage require two?