  Ground-based simultaneous multiband transit observations allow an accurate
system parameters determination and may lead to the detection and
characterization of additional bodies via the transit timing variations (TTVs)
method. We aimed to (i) characterize the heavily bloated WASP-4b hot Jupiter
and its star by measuring system parameters and the dependence of the planetary
radius as a function of four (Sloan g', r', i', z') wavelengths and (ii) search
for TTVs. We recorded 987 images during three complete transits with the GROND
instrument, mounted on the MPG/ESO-2.2m telescope at La Silla Observatory.
Assuming a quadratic law for the stellar limb darkening we derive system
parameters by fitting a composite transit light curve over all bandpasses
simultaneously. To compute uncertainties of the fitted parameters, we employ
the Bootstrap Monte Carlo Method. The three central transit times are measured
with precision down to 6 s. We find a planetary radius Rp = 1.413+/-0.020 RJup,
an orbital inclination i = 88deg57'+/-0.45deg and calculate a new ephemeris, a
period P = 1.33823144+/-0.00000032days and a reference transit epoch T0 =
2454697.798311+/-0.000046 (BJD). Analysis of the new transit mid-times in
combination with previous measurements shows no sign of a TTV signal greater
than 20s. We perform simplified numerical simulations to place upper-mass
limits of a hypothetical perturber in the WASP-4b system.
