  This is the addendum to the paper "On the Multiplicity Problem and the
Isomorphism Problem for the Four Subspace Algebra" Communications in Algebra,
40:6 (2012), 2005-2036 (DOI: 10.1080/00927872.2011.570830). We give here the
full proof of Proposition 3.3, describing the formulas for the dimensions of
the homomorphism spaces to indecomposable modules over the four subspace
algebra \Lambda. The reader can also find here the full description of
indecomposable \Lambda-modules.
