USA 4 will have six spinning yellow letters, as shown in the achievement itself:
Collect all six "B-U-R-G-E-R" letters within a single playthrough of a level, and once you've collected all six, you'll get the achievement. It might require some jumping and waiting for platforms to rise, so definitely take advantage of using the pepper and the spatulas to knock enemies out of the way.
You will need to complete this to get the following achievements anyway:
Good luck!