How to make my app an CI4 from CI3?

Hello community, I am a developer who is of the old CI3, I would like a tip that I should learn to adapt my CI3 projects to adapt it to CI4. I simply ask this user who seeks to update their applications and does not understand much it is always very useful to understand that I must learn to adapt everything to the new framework.

From now on community I await your prompt response cordial greetings to all.

[Image: https://i.ibb.co/9t33pTc/hexomecloud.jpg]