The Metolius 5 Step Aider is the best etrier-style aider we have tested. Chris McNamara has used this aider on more walls than any other aider and says it is comfortable and easy to walk up in due to its reinforced Biothane step. It is also the best aider at top-stepping of all the aiders we have tested because of the many sub steps. The only downside is durability, which only matters if you climb a ton of walls. Also, it is not quite as comfortable as the ladder-style aiders with spreader bars.
This review of the 5 Step Aider also applies to the 4 Step Aider. The only difference between the two is that the 5 step has additional step that adds 12 inches to the length. Chris normally uses the 4 Step except when doing hard aid, where the 5 Step is better. Also, if you are taller than 6 feet, you might prefer the 5 Step for all applications.