In the framework of the LHC Injector Upgrade program, CERN Linac4, a 160MeV H- ion linac, has replaced the 50 MeV proton linac (Linac2) as injector to the PS Booster since December 2020. The installation of the machine has proceeded in parallel with a staged beam commissioning at the energies of 3, 12, 50, 100 and finally 160 MeV, in 2016. Two reliability and beam quality runs took place during 2017 and 2018 to assess potential weak points and find mitigations. The lessons learnt during its construction and the main outcomes of the beam commissioning are presented in this talk.