Why the hell is Etymotic selling the ER-4S version when the ER-4PT is the one that doesn't have the 75Ω resistor embedded in the barrel. At least then you'd be able to use it with your smartphone. It's not like it makes any difference in cost and is probably actually cheaper for Etymotic but I expect they are probably Scrooge McDuck'n or hopefully it was just a mistake and they meant the ER-4PT. I would hope that it is the latter.
After all these are the people that will sell a cable adapter with a 75Ω resistor in series for $55 to convert your PT to S. Rip off is an understatement and others sell these adapters on ebay for $15.
The green filters from Etymotic are also really expensive considering how cheap you can get them on mouser. Considering that in the lifespan of an IEM you'll be buying replacement tips you'll certainly be charged quite a lot for them by Etymotic. Etymotic are certainly happy to take you for a ride for their overpriced accessories.
The cable it comes with is also awful and hasn't been updated in 20 years. Super thick rubber below the split and microphonic as hell above. The cable is simply awful for portable use which is how the majority of buyers will use it considering that it's an IEM with excellent noise attenuation. You will have to buy an aftermarket cable to get something sensible and they are pretty damn expensive because Etymotic use their own proprietary connector that nobody else uses. You can get adapters that convert the Etymotic conector to a MMCX or standard 2 pin but these adapters are pretty bulky and stick out the ear.
You start to get the picture of how much Etymotic cares. Still as an IEM goes they are pretty damn good though most people prefer some bass boost for enjoyable listening.
You can also use a 75Ω with the HF series: http://rinchoi.blogspot.com/2013/04/etymotic-research-hf-3-er-4-equivalent.html
However, the treble isn't boosted as much and the HF5 that Rin Choi tested appeared to have more bass roll off than the ER4 they tested.
I would recommend getting foam tips for comfort and better noise attenuation. The foam tips that Etymotic IEMs come with is a pretty rough and abrasive foam. Comply foam is good but SO EXPENSIVE.
You can change the filters to change frequency response as well: http://rinchoi.blogspot.com/2012/08/the-effect-of-acoustic-dampers-on-iems.html
Etymotic only provide green. Many like orange/red/yellow which reduces the mids for a bassier sound.
You can find them for sale here: https://mouser.com/search/refine.aspx?Keyword=knowles+dampers&FS=True
You can work out which is which from the impedance.
@EdinNJ
Custom tips certainly are more comfortable but the shallow insertion will mean that it will change the frequency response and increase the higher frequencies due to the occlusion effect. For the reference frequency response they need to be inserted to the 'reference plane' which is around the second bend in the ear canal and I think it's defined as a certain volume of air... I think it is 0.5cc (0.5ml).
It is explained here: http://rinchoi.blogspot.com/2010/05/how-deep-do-you-insert-you-er4.html
You can see in the graph that with a shallow insertion you'll get a spike in frequency response at 10Khz.
Personally, if you don't care about the reference sound I'd just buy the HF2/3/5 which is more than $100 cheaper. Buy a 75Ω P to S adapter from ebay if you want to boost the treble a bit. Buy some acoustic filters and have fun. The HF series is only very slightly worse but has an inline mic and a nicer (certainly not the best) cable.
https://www.massdrop.com/buy/etymotic-er4-s-micro-pro-earphones/talk#!233537