If you set the hook option to only send 400 bulk emails in an hour then it will send a maximum of 400 bulk emails in an hour.
The hook still honours the per cycle setting too, so if you set this to 100 then the mails will be sent in chunks of 100 until 400 have been sent in an hour.
The hook does not change how the bulk mailer task is executed, it simply restricts the number of bulk mails sent in a given interval.