A bit of a pre-flight checklist that you need to go through first and the first

• Confirm that any plugins you're using are available for Craft 4.
• Update the Craft 3 Site to the latest version.
• Update the plugins to the latest version.
• Fix all deprecation warnings. You'll find a list of warnings in admin panel under utilities, deprecation warnings.
• Make sure you don’t have any pending or active jobs in your queue.

1. Run php craft project-config/rebuild and allow any new background tasks to complete.
2. Update DDEV php to 8.1 and add "php": "8.1" to composer.json.
3. Remove aelvan/imager plugin ~ ddev composer remove aelvan/imager.
4. Edit your project’s composer.json to require "craftcms/cms": "^4.0.0".
5. Run composer update.
6. Make any required changes to your configuration.
7. Run php craft migrate/all.

YOU'RE NEARLY DONE

8. Upgrade vlucas/phpdotenv plugin to 4.0.
9. Edit your project’s index.php, replace:

(new Dotenv\Dotenv(CRAFT_BASE_PATH))->load(); with

    $dotenv = Dotenv\Dotenv::createMutable(CRAFT_BASE_PATH, '.env'); 
    $dotenv->load();

10. Replace all {%- spaceless %}, with {%- apply spaceless %}.
11. Replace all {%- endspaceless %}, with {%- endapply %}.
12. Edit templates/_macros.twig, isEditable is not defined in craftcms 4, use currentUser and entry is defined.
13. Twig 3 removed support for the if param in {% for %} tags, they suggested you use |filter instead. But you can just add an if statement inside the {% for %}.
14. Edit templates/_includes/form.twig, replace {% for field in tab.getFields() %} with {% for field in form.getFieldLayout().customFields %}.