It is known that, to avoid the occlusion effect of IEMs(including custom-moulded ones) and hearing aids, you should fill up your external ear canal till the 2nd bend(approximately 1/2 of the external ear canal length), where the bony part is. Furthermore, that's very likely the depth you can achieve the tonal accuracy of ER4. (and freedom from lack of bass)
[Industry-standard occluded ear simulators]
[IEC 60318-4(formerly IEC 60711) & ANSI/ASA S3.25 ]
Manufacturers of insert earphones use above ear simulators to measure, and fine-tune their products. These simulators are even implemented in Head-And-Torso-Simulators(aka dummy heads) too. Insert earphones are measured as shown below:
"An essential concept is that of the reference plane. This is a plane, at right angles to the longitudinal axis of the ear canal, located at the point in the ear canal where the earmold or ear shell usually terminates (defined in the standards to be approximately 13mm from the ear drum). An ear simulator (and very approximately, a coupler) represents the acoustic impedance of the residual ear canal volume and middle ear from this point inward."
Dillon, H, Hearing Aids. Sydney, Australia: Boomerang Press, 2001.
IEC 60318-4 and ANSI/ASA S3.25, this residual volume is defined as 0.547cc and 0.558cc each. Considering *the average volume of an ear canal with ∅7mm x 26mm, this residual volume can be achieved by filling up about the half of the ear canal. According to
*1cc =26 x pi x 3.5^2
As seen in this CT scan, the 2nd bend is just at the center of the ear canal, and it can be assumed that's the depth 0.5cc can be achieved approximately.
Full insertion depth is a must for ER-4 (or any IEMs calibrated in a similar way), otherwise you'll end up with a linear distortion such as above. On IEC 60138-4 & ANSI/ASA S3.25 simulators, λ/2 resonance occurs at ~*13kHz, and this acoustic impedance 'pulls' the earphone response at the high frequency range. Shallow insertion causes this interaction to occur at lower frequencies, and consequently will cause a sibilance and tin-can like timbre, especially on Etymotic Research ER-4B and Shure E4C. Above graphs show the effect of full insertion & **insertion 4mm short.
I actually built a DIY Zwislocki coupler sometime ago, and was able to further confirm the above. Therefore, I'd recommend:
Insert your ER4 upto the 2nd bend, or until you can not hear the resonance of your own voice.
(to be more precise, the residual canal volume of 0.5cc)
(Same principle also applies to other IEMs calibrated/manufactured in reference to 2cm^3)
On Nov. 18th, 2010, Mr. J. Stewart from Etymotic Research provided further insights: