Limitations for sending emails
At Administer > CiviMail > Mailer Settings (civicrm/admin/mail) you will find settings for CiviMail delivery.

Do your settings have a Mailer Batch Limit of 400? If so, this was probably set on your site at some point.

The default is 0 (unlimited) I think; if it was set to 400 on your site this may indicate the task was timing out before completion. If you're having that issue, running the mail task more frequently may be a good alternative to adjusting the batch limit.