Keep in mind this is for the hardware work. We have been working on the HL-LHC from the physics / beam dynamics / everything else side for close to a decade now.

The other machines in the complex, which we call "injectors" from our mighty tower, have already undergone their own upgrade to be ready for HL-LHC, the LIU (https://home.cern/accelerator-chain-prepares-high-luminosity...).

Nonetheless there are plenty of things to be done and a lot of technologically engaging challenges ahead so this 3-4 years is still ambitious ;)