In the framework of the LHC Injector Upgrade program, a new normal conducting linac (Linac4) operating at the frequency of 352 MHz has been recently commissioned to the final energy of 160 MeV. Linac4 will be connected to the LHC injector chain in 2020 and it is expected to provide a 40 mA, 400 µsec H$^-$ beam for charge-exchange injection into the Proton Synchrotron Booster.
The Linac4 pre-injector includes a 45 keV, 2 MHz rf-driven, cesiated H$^-$ source, a magnetic Low Energy Beam Transport and a 3 MeV Radio Frequency Quadrupole. Currently the pre-injector is limited to beam currents of 35 mA.
A campaign of beam measurements at 45 keV under controlled variation of the system parameters has been performed during 2017 to improve the understanding of pre-injector dynamics and to devise a method to overcome the present limitation. This paper reports the results of the measurements and preliminary conclusions on the dependence of current and emittance on the relevant parameters.