Here is a guide on how to slightly modify your ER4 custom IEM tips to make them sound much better.
Some others on Head-Fi had mentioned decreases accuracy and shrill sound after installing the custom tips on their etymotics.
I recently bought a pair and also noticed a dramatic change in sound. I would personally describe it as if someone had put hand over my ears and decreased the treble and made it shrill.
Symbols and S sounds had a harsh TSSSSSS quality which I have never encountered with over a decade with this headphones with various tips.
Needless to say, I was not very impressed.
I came to the conclusion that either the bend in the sound bore of the molds was modifying the sound or the fact that the volume of air in the bore now was greater than that of other tips.
Notice the steep bend in the sound bore
The solution to this problem is to remove the filters from the tip of the headphone to the tip of the mold.
In this way, the sound is not filtered until it gets closer to the eardrum.
I also guess that the sound has more power with the filter removed and therefore can easily move the air in the sound bore before the filter.
Whatever the effect of this modification, the results speak for themselves with my ER4 sounding better than I have ever heard it.
Step 1: Remove Filters From Earphone
Use the filter removal tool to remove the filter from the headphone.
This will destroy them, so you will need 2 fresh filters after this step.
You may want to remove the green fabric from the center of the filter and replace the empty filter "can" into the headphone.
Step 2: Find Some Pens, Remove Ink Tube
Gather a few pens and pull the ink inserts out of them.
Compare the opening of the pen insert to the filter size.
Pick a tube which is slightly smaller than the filter.
Notice that the tube to the left is a better fit than the tube to the right which may be too small
Step 3: Cut Tube
Cut the end of the tube so it is slightly longer than the filter.
Step 4: Insert Filter
Insert filter into tube.
Turn tube upside down and press filter in.
Step 5: Insert Into Mold
Let me know how this works for you and if it improves sound any