  CONTEXT. Exoplanet searches have demonstrated that giant planets are
preferentially found around metal-rich stars and that their fraction increases
with the stellar mass. AIMS. During the past six years, we have conducted a
radial velocity follow-up program of 166 giant stars, to detect substellar
companions, and characterizing their orbital properties. Using this
information, we aim to study the role of the stellar evolution in the orbital
parameters of the companions, and to unveil possible correlations between the
stellar properties and the occurrence rate of giant planets. METHODS. Using
FEROS and CHIRON spectra, we have computed precision radial velocities and we
have derived atmospheric and physical parameters for all of our targets.
Additionally, velocities computed from UCLES spectra are presented here. By
studying the periodic radial velocity signals, we have detected the presence of
several substellar companions. RESULTS. We present four new planetary systems
around the giant stars HIP8541, HIP74890, HIP84056 and HIP95124. Additionally,
we find that giant planets are more frequent around metal-rich stars, reaching
a peak in the detection of $f$ = 16.7$^{+15.5}_{-5.9}$% around stars with
[Fe/H] $\sim$ 0.35 dex. Similarly, we observe a positive correlation of the
planet occurrence rate with the stellar mass, between M$_\star$ $\sim$ 1.0 -2.1
M$_\odot$, with a maximum of $f$ = 13.0$^{+10.1}_{-4.2}$%, at M$_\star$ = 2.1
M$_\odot$. CONCLUSIONS. We conclude that giant planets are preferentially
formed around metal-rich stars. Also, we conclude that they are more
efficiently formed around more massive stars, in the mass range of M$_\star$
$\sim$ 1.0 - 2.1 M$_\odot$. These observational results confirm previous
findings for solar-type and post-MS hosting stars, and provide further support
to the core-accretion formation model.
