If I understand correctly you need 6,000 Membership Rewards points to redeem for four Rapid Rewards credits? If you have a Gold, Platinum or Centurion card American Express will advance you more than 6,000 points, and you will have a year to earn them.
Alternately you can buy Membership Rewards points for $25 per thousand.
To learn about these features start here and read the paragraph titled "Earn Rewards Faster"...
http://www.membershiprewards.com/hel...ge.aspx?t=faqs
I don't recall if Green cardholders participate in Points Advance, but they certainly can purchase points.