How do I limit the amount of emails sent in a campaign?

Here's the situation; We want to send a message to 80,000 toal contacts, but send 20,000 per hour at the top of each hour for 4 consecutive hours. This is to control the flow of inbound leads in this direct inquiry campaign in order to enable our reps enough time to quickly respond to each with a folow-up phone call.

I figured this would be a pretty simple thing to do, but seem to be missing how to set this limit.

Any ideas?