Upgrade from CI3 to CI4 - By Running them together

what about router ? how run router CI3 and CI4 in the same project

No Changes in CI3 Routing.

CI4 add the route like

$routes->get('welcome/index', 'Home::index');

And in CI3 welcome controller, index function comment all the lines of code and add $this->load_from_ci4();

This should load the CI4 Home controller, index function