Mozart

[Spotted Horses]

Of course not. The title is a mystery to me as well.

Four orchestras is an obvious reference to the Notturno K286. Maybe they combined the forces of the two PI ensembles listed for a grand ensemble to perform the other two symphonies? We need someone who had the CD and can read the booklet for clues.

[Florestan]

Four orchestras is an obvious reference to the Notturno K286. Maybe they combined the forces of the two PI ensembles listened for a grand ensemble to perform the other two symphonies? We need someone who had the CD and can read the booklet for clues.

The most probable explanation is that the two orchestras were combined for S34 and S40 and split for KV 286. 

[Jo498]

I have that Brüggen disc. It's just a fairly big orchestra in the symphonies, including probably doubled winds in the tutti, put together from the two named ensembles for the symphonies.
So orchestra in the CD title is used ambiguously: 2 orchestras play music for 4 orchestras (and for only 1) 
There is actually no further information about the exact strength of the combined orchestra for the symphonies. Simply all members of both are named but I doubt that the doubled the timpani for K 338, so probably only one of them is playing.

There are a few "double orchestra" symphonies by JC Bach but I don't know how they are set up.