MySQL 4 byte is now detected during setup in any case.

config.sample.php was updated to explicitly state that there detection in place to set mysql.utf8mb4

config/config.sample.php

@@ -1234,8 +1234,12 @@ $CONFIG = array(
 'sqlite.journal_mode' => 'DELETE',
 
 /**
- * If this setting is set to true MySQL can handle 4 byte characters instead of
- * 3 byte characters
+ * If requirements are met (see below) this setting is set to true during setup
+ * and MySQL can handle 4 byte characters instead of 3 byte characters.
+ *
+ * If you want to convert a 3-byte setup into a 4-byte setup please run the
+ * migration command:
+ * ./occ db:convert-mysql-charset
  *
  * MySQL requires a special setup for longer indexes (> 767 bytes) which are
  * needed:

lib/private/Setup/MySQL.php

@@ -38,12 +38,10 @@ class MySQL extends AbstractDatabase {
 		$connection = $this->connect(['dbname' => null]);
 
 		// detect mb4
-		if (is_null($this->config->getValue('mysql.utf8mb4', null))) {
-			$tools = new MySqlTools();
-			if ($tools->supports4ByteCharset($connection)) {
-				$this->config->setValue('mysql.utf8mb4', true);
-				$connection = $this->connect();
-			}
+		$tools = new MySqlTools();
+		if ($tools->supports4ByteCharset($connection)) {
+			$this->config->setValue('mysql.utf8mb4', true);
+			$connection = $this->connect();
 		}
 
 		$this->createSpecificUser($username, $connection);