  The gaseous giant planets WASP-4b and WASP-5b are transiting 12 magnitude
solar-type stars in the Southern hemisphere. The aim of the present work is to
refine the parameters of these systems using high cadence VLT/FORS2 z-band
transit photometry and high resolution VLT/UVES spectroscopy. For WASP-4, the
new estimates for the planet radius and mass from a combined analysis of our
VLT data with previously published transit photometry and radial velocities are
R_p = 1.30 +0.05-0.04 R_jup and M_p = 1.21 +0.13-0.08 M_jup, resulting in a
density rho_p = 0.55 +0.04-0.02 rho_jup. The radius and mass for the host star
are R_s = 0.87 +0.04-0.03 R_sun and M_s = 0.85 +0.11-0.07 M_sun. Our
ground-based photometry reaches 550 ppm at time sampling of ~50 seconds.
Nevertheless, we also report the presence of an instrumental effect on the VLT
that degraded our photometry for the WASP-5 observations. This effect could be
a major problem for similar programs. Our new estimates for the parameters of
the WASP-5 system are R_p = 1.09 +-0.07 R_jup, M_p = 1.58 +0.13-0.10 M_jup,
rho_p = 1.23 +0.26-0.16 rho_jup, R_s = 1.03 +0.06-0.07 R_sun, and M_s = 0.96
+0.13-0.09 M_sun. The measured size of WASP-5b agrees well with the basic
models of irradiated planets, while WASP-4b is clearly an `anomalously' large
planet.
