[throzen0303]

Since the ER4P is basically the same thing as a 4S with lower impedance, does it mean for most earphone, adding impedance would make the sound better/change? 
 
 

 

[umvue]

If you use the logic further, then you might wonder will ER4P sound even better if you add 275 ohm to it.I think it is unlikely to better because the IEM is not built for 300 ohm. ER4P is very likely to be built with 100 ohm in mind. For other IEMs that are not built for higher impedance, I believe the advantage will be minimal. There might be some other IEMs that are built for higher impedance than spec'ed but you have to experiment them yourself.
 
Again, it is just my further logical extension, it might not conform to the reality.

 

[Eric_C]

I don't think adding or reducing impedance necessarily makes a transducer sound better; it's more about having the appropriate impedance for the setup.
 
Tyll discusses this in relation to damping factor (which, by his own admission in the comments, is a simplified version of things) in this review of the DT 880 and its 3 impedance versions:
http://www.innerfidelity.com/content/comparison-beyerdynamic-dt-880-32-ohm-dt-880-250-ohm-and-dt-880-600-ohm-headphones