Manual Install

I prefer not to use the .env file. Without the leading period the env file settings are inactive.

Also as a temporary override I use the following:
File: index.php

Code:

<?php // declare(strict_types=1); // unfortunately CI4 fails validation

// Bypass htaccess setting of CI_ENVIRONMENT
// $_SERVER['CI_ENVIRONMENT'] = 'production';  // bypass .env & .htaccess
   $_SERVER['CI_ENVIRONMENT'] = 'development'; // bypass .env & .htaccess

// bypass (bool) app/Config/boot/development.php
   define('CI_DEBUG',  FALSE);
   error_reporting(-1); // regardless
   ini_set('display_errors', '1');

// disable Klint and prevent overriding page layout
// require '../system/ThirdParty/Kint/kint.php';
   $useKint =  FALSE;

  // remaining index.php content goes here
   $useKint =  FALSE;

and as @includebeer mentioned:
Don't forget to check the log files in the “writable” folder.