I have a set of Etymotic ER 4 with custom fit molds of my ears.
 
I wanted to use these molds and try different Earphones / Monitors.
 
I can't seem to find a specification on the opening of the Etymotics.
 
I went to some of the sites that offer universal replacement tips, and cross referenced a few other headphones using the same universal tips as the ER 4's, but before I drop some money on other Earphones, I wanted to be sure.
 
Has anyone here tried what I'm suggesting?
 
The opening on the ER 4 seems to be much smaller of any of the other monitors I own like my Beats Studio, Sleek Audio and V-Moda.
 
Thoughts?

The opening on your ER4 will be on the small side, with a smaller nozzle than most dynamic driver IEMs. DD IEMs are usually 5-6mm in nozzle width; I would say Etymotic is around 2-3mm (I don't know the exact figure).
 
They sell 'step-down' adapters to use one size tip across multiple nozzle sizes, but it might be a new product and I don't know how to find it; I've only heard of it in passing. Luckily quite a few single BA IEMs use the smaller nozzle size, but it doesn't help matters that the ER4 is probably the best among them.