I still believe that the fifth horcrux is Riddle's trophy -- the one Ron barfed on.

It fits for a lot of little reasons.

My main reason is that I don't think Jo will introduce any more new stuff than
is absolutely necessary. She's spent a really, really long time setting up this
elaborate series of dominoes, and I think that in the last book she will just tap
that last domino, sit back and watch it happen.