(S4 vs. S4R model)

You can find a ton of videos and blogs on the topic of open vs closed back headphones. I'll try to explain general difference and dive deepen into differences between the S4 and S4R headphones.

The open back headphones are a better choice for mixing, while the closed back are primarly designed for tracking and recording sessions.

Open headphones are designed to sound more natural and closer to standard monitoring situations while the closed back are primarly designed to isolate external voice and leakage of sound from the headphone out into the environment.

Closed back headphones

Isolation of leakage of sound from the headphone into the environment is super important when you're recording acoustic instruments or vocals. Usually the click track will escape almost any consumer headphone insulation and get into the microphone. So for recording vocals or acoustic guitars you'll need to have a nice pro audio closed back headphones.

What to look for when buying closed back headphones?

You can find a bunch on the market from less than €100 to about €400. Anything more than this is a luxury that does not make a reasonable purchase. The added quality in sound, comfort or durability will not outweigh the price difference. If you are a studio owner and you need like 10 or more closed back headphones for choir recording or other larger groups than I'd recommend you go with Sony MDR 7506. The sound is not reference but the accuracy is excellent, and the price is at around €100/pcs.

Why owning a pair of excellent quality closed back headphones matters anyway?

Sometimes you'll get a vocalist in, and she'll tell you that her voice sounds different than what she's used to. That will affect her performance. This, plus the fact she'll expect pro headphones in a pro studio, something she doesn't find in her home studio. That's why I'd recommend every studio to have at least one good quality closed back headphones. The sound of S4R or similar models by Focal or Adam Audio is something you need to offer to a pro level artist. So at least a pair is a must in my opinion.

Open back headphones

Downsides of open back headphones

The open back headphones are mainly used for mixing or referencing tracks in post-production sessions. They leak the sound out of them, and that can be annoying if you're using them in a coworking environment or in your home studio that is also a living room for your family. It goes the other way too. Any noise from the environment will get into the headphone unless you have them at higher volumes to mask it, which is not a great idea as it can hurt your ears. Studies show that at about 80dB you can only listen up to 9 minutes without hurting your ears and going into ear fatiguing moments.

But that being said, the mixing experience on headphones can only come close to standard monitoring situations with open back headphones.

OLLO Audio S4 are designed especially for mixing engineers

There's a lot of open back consumer headphones but not much in the mixing - pro audio area. One of the most popular is the Sennheiser HD650. If you're really into mixing on headphones due to necessity or you learned that it's possible to do great mixes and directing all your effects on them, then I'd recommend going with the S4 mixing headphones. They are designed for mixing engineers. The comfort will enable you to mix for 5 hours straight. The frequency response is designed to be flat out of the box so no need for any software correction. Also, the acoustics design of S4 was developed in a way to support minimal changes in SPL. So you'll hear a 1-2dB compression instantly.

OLLO Audio S4 vs. S4R

In comparison to S4, the S4R get a bit louder due to closed back design before distorting, and their frequency response is less flat. That comes with any closed back headphones. Due to the closed cabinet, there will be a resonance frequency that will boost as some spectrum range and dip right after it. In S4R that the 500Hz area. You'll find this to be at different spectrum ranges for different headphones. You can see the difference in the graph below. 

Here is a nice video by Sean Divine explaining differences using the OLLO Audio S4 as open back and Adam Audio SP-5 as closed back headphone.