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.
It's been in the planning stages for at least 5-8 years at this point. The instruments and upgrades have been tested and validated so it's just putting stuff together and then validating/calibrating. Stuff can still happen like the splice issue and repairs.
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 ;)