4 people x 8 credits x 2 ways = 64 credits for A+ Reward or Standard Award tickets.
128 Credits will get you 4 round trip A+ Biz tickets.

You will need to wait to see what flights are like and whether booking on Airtran.com is even an option. Southwest has been taking over Airtran flights and once a flight becomes fully Southwest it can only be booked on Southwest.com. You may find Airtran availability limited.

That said, booking on Southwest.com does have certain advantages like being able to cancel the flight and get your credits back, no bag fees and no change fees.

Also any Standard Awards created on Southwest.com expire one year from the date of creation, possibly extending the life of your credits.

The primary advantage of booking on Airtran.com is being able to book Biz tickets which will probably be more available especially at high travel times like Labor Day. You might want to look at blackout dates which have already been published.

Finally I would recommend getting your plan in order and booking as soon as the schedule is extended on March 3. Holiday weekends fill up fast.