Context : 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 .

Aims : 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 ^ { \prime } , r ^ { \prime } , i ^ { \prime } , z ^ { \prime } ) wavelengths and ( ii ) search for TTVs .

Methods : 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 .

Results : The three central transit times are measured with precision down to 6 s. We find a planetary radius R _ { \mathrm { p } } = 1.413 \pm 0.020 R _ { \mathrm { Jup } } , an orbital inclination i = 88. ^ { \circ } 57 \pm 0.45 ^ { \circ } and calculate a new ephemeris , a period P = 1.33823144 \pm 0.00000032 days and a reference transit epoch T _ { \mathrm { 0 } } = 2454697.798311 \pm 0.000046 ( BJD ) .
Analysis of the new transit mid-times in combination with previous measurements shows no sign of a TTV signal greater than 20 s. We perform simplified numerical simulations to place upper-mass limits of a hypothetical perturber in the WASP-4b system .

Conclusions :