I have also thought about the Fifth Symphony – some sources mention "five symphonies" but I have never came across anything concrete.
One possibility is actually that by mistake (more or less), Maliszewski's contribution to the 1928 Schubert symphony competition might have been counted as "Symphony No. 5". Indeed, he wrote two movements for this (well-known, of course) competition, a Scherzo partially based on Schubert's sketches and a finale, i.e. he was one the composers who really tried a (sort of) completion of the Unfinished. They were finally labeled 'Scherzo and Overture in Honour of Schubert' and they are also included in this disc set (in particular the finale is an oddity which I find mildly amusing). It does not sound completely unreasonable to me that this is the work which is meant when some sources mention a Fifth Symphony but of course this is just a guess.
One possibility is actually that by mistake (more or less), Maliszewski's contribution to the 1928 Schubert symphony competition might have been counted as "Symphony No. 5". Indeed, he wrote two movements for this (well-known, of course) competition, a Scherzo partially based on Schubert's sketches and a finale, i.e. he was one the composers who really tried a (sort of) completion of the Unfinished. They were finally labeled 'Scherzo and Overture in Honour of Schubert' and they are also included in this disc set (in particular the finale is an oddity which I find mildly amusing). It does not sound completely unreasonable to me that this is the work which is meant when some sources mention a Fifth Symphony but of course this is just a guess.