  This work concerns the non-flat metrics on the Heisenberg Lie group of
dimension three $\Heis_3(\RR)$ and the bi-invariant metrics on the solvable Lie
groups of dimension four. On $\Heis_3(\RR)$ we prove that the property of the
metric being naturally reductive is equivalent to the property of the center
being non-degenerate. These metrics are Lorentzian algebraic Ricci solitons. We
start with the indecomposable Lie groups of dimension four admitting
bi-invariant metrics and which act on $\Heis_3(\RR)$ by isometries and we
finally study some geometrical features on these spaces.
