  It is known that some two qutrit entangled states of rank 4 with positive
partial transpose [PPT] can be built from the unextendible product bases [UPB].
We show that this fact is indeed universal, namely all such states can be
constructed from UPB. We also classify the 5-dimensional subspaces of two
qutrits which contain only finitely many product states (up to scalar
multiple), and in particular those spanned by a UPB.
