Cakephp version 2.6 to 4.0

I am currently working with version 2.6 and want to update to 4.0. I am looking for advice on upgrading to this version from experience. For those who have upgraded what were the most important steps you took that saved you time and a giant headache.

Have you searched through past topics here? This comes up fairly often, so there should be lots of good answers already.