In the old days if you wanted a guaranteed 4000 IOPS on EBS, you had to provision a minimum of a 400 GB vo0lume. Considering you pay per the GB, and provisioned IOPS are not cheap, if you only needed 100 GB of fast storage you were stuck paying for 300 GB of unused storage.
With this recent announcement Amazon has made it easier to get fast storage in smaller increments. Now if you want 4000 IOPS you can get that in EBS volumes as small as 133 GB up to 1 TB in size. Read the following press release for more information.