As far as I'm concerned, some pieces do not need a change in
instrumentation. Take for example Chopin's nocturne in Eb op. 9 no. 2. I
heard an arrangement of this nocturne with a brass sextet. It wasn't very
interesting at all. A lot of pieces written for solo piano (especially
Chopin's) need not be transcribed. On the other hand, a lot of pieces
written for orchestra might sound good on 4 hand piano. (Lizst's
transcriptions)
Jeremey