Have a look at INNODB and hot copy. It's a rolling backup. Using mysqldump 4 times a day will kill your site (obviously). You're locking the tables while mysqldump dumps over a million posts. That probably takes 2 hours or more. 4 times a day. So, for 8 hours a day your database is locked. No wonder it's dead most of the time. Look into alternate methods of backing up this data. Seriously.
Actually, you can run mysqldump without locking the tables:
mysqldump -uusername -ppassword --skip-add-locks --skip-lock-tables dbname > db.sql
However, I must agree that 4 times a day is way too much. I would do only a nightly backup.
