  We have studied the magnetic properties of quasi two-dimensional itinerant
ferromagnet Fe$_4$GeTe$_2$ as functions of temperature and magnetic field ($B$)
with field parallel to the $ab$-plane ($B\|ab$) and $c$-axis ($B\|c$) of the
crystal. This van der Waals compound undergoes a continuous paramagnetic to
ferromagnetic phase transition below $T_C$ = 270.0 K and a pronounced spin
reorientation transition at around $T_{SR}$ $\sim$ 115 K where the easy axis of
magnetization changes its direction from in-plane to out-of-plane. The
temperature evolution of magnetization in the FM state is highly anomalous and
extremely sensitive to the direction and strength of applied magnetic field.
Magnetic entropy change ($\Delta S_M$) has been estimated in the vicinity of
$T_C$ and $T_{SR}$. $\Delta S_M$($T$) is found to be almost isotropic around
$T_C$ while it shows very unusual behavior and is sensitive to the direction of
applied field at low temperature close to $T_{SR}$. For $B\|c$, $\Delta S_M$ is
negative and decreases continuously with field and -$\Delta S_M$($T$) shows a
broad maximum around $T_{SR}$ similar to that observe at $T_C$. On the other
hand, $\Delta S_M$ is positive in the vicinity of $T_{SR}$ below 1.4 T and
-$\Delta S_M$($T$) exhibits a peak at $T_{SR}$ which becomes very sharp at high
field for $B\|ab$. These results suggest complex nature of the magnetic ground
state of Fe$_4$GeTe$_2$, which is possibly due to the presence of multiple
inequivalent Fe sites, their ordering and complexity of spin configuration.
