That's a great question you've raised. The short answer is: yes - The A/B or Split Test will spend 4 hours analyzing the open rates for the subject lines you're testing to 20% of your audience before sending out the most successful subject line to the remaining 80% of your audience. The idea behind this length of time is that it gives those mailings a good chance to be opened if they are going to be opened at all.
The good news is you can speed this process up if you like. You'll see when you open up the split test from your response page, you'll have the option next to each bar graph signifying the subject line to end the split test and send that subject line to the remainder of the audience.
Although A/B testing or Split testing are a great way to determine what's working and what's not in regards to getting folks to open your mailings, if you're sending something out that's quite time-specific it might be better to avoid this due to the delay.
I hope that helps Loren -- please let me know if you have any more questions on this :)