er4b is airy because it has a treble shelf eq baked in by the hardware. er4s and er4b are identical earphones with different electrical components that shelf the trble in different amounts.
the er4p has a small resistor, the er4s has a larger resistor that increases the treble. the er4b adds a capacitor rc type circuit i believe to further increase the treble. that's why you can buy and er4 and swap cables to change the model you have.
in that regard, the er4sr is almost the same response as the er4s, but you can't change cables like you used to. so the er4b should be noticeably brighter (it is to me). airy, yes, but most people will find it thin and lacking bass. it was geared towards binaural recordings (hence the b moniker) and does super on those, but for common music it is probably not ideal, although if you like there's nothing wrong with that. but it won't be accurately portraying the tonal balance. now you could just eq in some lows and have a slightly u shaped etymoic sound. but I'd opt for the s myself or sr.
and i agree that the s with eq sounds better than the xr variants. but without eq i prefer the xr most of the time as the sr is more dynamic and transparent sounding, but lacks any low end that give the realistic bass response. you can hear it fine, but tonally it doesn't sound speaker like or accurare from a mixing engineers point of view...
I'm still waiting dor that perfect dynamic driver etymotic somke day. for now, despite maybe being a touch dark in some way, the er2xr has the most natural sound to me. er4xr has better response, but sounds less natural.