Here is an article that details my path to upgrade to Slimframework v3 to v4. I'm developing on my freetime an application that manage the fundraising of the French Red Cross, and I'm updating a lot of frameworks to their last version (AngularJS to Angular 8/9 is my big next move). You can find the project sources here : https://github.com/dev-mansonthomas/RedCrossQuest There’s quite a lot of breaking changes involved, feature removed in profit to external frameworks, the upgrade documentation is quite minimal, but you must read it to understand what's changing : http://www.slimframework.com/docs/v4/start/upgrade.html This article is not well redacted, it’s just a log of what I did to achieve the upgrade, but I feel it may help some, so here we are ;) What I've read to perform the upgrade I’ve read theses article to help the transition to the 4.x https://akrabat.com/a-first-look-at-slim-4/ https://dzone.com/articles/a-first-look-at-slim-4 ...