  In a previous memoir 2202.03030, we showed that in every dimension $n \geq
5$, there exists (unexpectedly) no affinely homogeneous hypersurface $H^n
\subset \mathbb{R}^{n+1}$ having Hessian of constant rank 1 (and not being
affinely equivalent to a product with $\mathbb{R}^{m \geqslant 1}$). The
present work is devoted to determine all non-product constant Hessian rank 1
affinely homogeneous hypersurfaces $H^n \subset \mathbb{R}^{n+1}$ in dimensions
$n = 2, 3, 4$, the cases $n = 1, 2$ being known. With complete details in the
case $n = 2$, we illustrate the main features of what can be termed the "Power
Series Method of Equivalence". The gist is to capture invariants at the origin
only, to create branches, and to infinitesimalize calculations. In dimension $n
= 3$, we find a single homogeneous model: \[ u \,=\, \frac{1}{3\,z^2} \Big\{
\big( 1-2\,y+y^2-2\,xz \big)^{3/2} - (1-y)\, \big( 1-2\,y+y^2-3\,xz \big)
\Big\}, \] the singularity $\frac{1}{3z^2}$ being illusory. In dimension $n =
4$, without reaching closed forms, we find two simply homogeneous models,
differing by some $\pm$ sign.
