Headphone impedance suggestion on most amp are just rubbish info that serves no real technical purpose. There are there for marketing only, because the actual measurement data that tells you how good an amp is is often too complicated for most to understand, so manufacturers come up with these simple number 'suggestion' system hoping the buyer can make quick and simple (but equally uninformed) choice. That being said, most half decent headphone amp should be able to driver ER4S well - it is far from the most difficult IEM to drive. What you want it low output impedance (not to be confused with headphone impedance manufacturer is suggesting) and decent enough output power (just a rough idea - you don't want anything less than probably 20mW into 16ohm load, more is better). All of FiiO's and iFi's amp should easily beat those requirement.