Steps to reproduce the issue
use normal mod_login in frontend
Expected result
works normally
Actual result
redirect to /component/users?task=user.login with error 404 (The requested URL /index.php was not found on this server.)
System information (as much as possible)
Joomla! 4.0.0-alpha3 Alpha [ Amani ] 12-May-2018 15:23 GMT
mysql 5.6.37
php 7.1.8
Apache/2.4.27 (Unix)
Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/11.1 Safari/605.1.15
localhost using AMPPS
Additional comments
I can not reproduce your issue. Here redirect normal to the home page.
Joomla! 4.0.0-alpha3 Alpha [ Amani ] 12-May-2018 15:23 GMT
php 7.2.4
Apache/2.4.33