LINAC4 is the first step in the upgrade of the injector chain for the LHC, and will accelerate H ions from 45 keV to 160 MeV. Currently the ion source is installed in a test setup and its commissioning started at the end of 2009. A slit-grid system is used to measure the transverse emittance of the beam at the exit of the source. The results of the measurements have been compared with analytical and numerical predictions of the performance of the emittance meter, addressing the system resolution, accuracy and sensitivity. The outcome of this analysis has been used to improve the design of a new slit-grid system foreseen for the commissioning of LINAC 4 at higher energy locations.