Hello,
I just tried to activate LF_APACHE_404 and LF_APACHE_403 but I activated and restart csf (csf -r and reboot) I does not blocked 404 floods.
I found a mistake in configuration file /etc/csf/csf.conf
HTACCESS_LOG = "/var/log/httpd/error_log"
MODSEC_LOG = "/var/log/httpd/error_log"
but it should be like below
HTACCESS_LOG = "/usr/local/apache/logs/error_log"
MODSEC_LOG = "/usr/local/apache/logs/error_log"
thats why, Csf not block 404, 403 and mod_security errors.