I have just ordered a set of replacement foamies from amazon as well. There are two types. One for the ER6i and one for the ER4. Here is a link to the two. They do look the same but I checked the other day as I have an old set of ones for my old er6's and they are different to the ones that came with my 4's!
Etymotic Research, Inc. - ER-6i Accessories
Etymotic Research, Inc. - ER-4? earphones
There is also one for the ER6(ER6-14F)
The ER 6 ones are noticeably different but the ones for the 6i's and 4's look very similar, the difference from what I can see is the 6i's have a smaller diameter bore in the plastic core. Could this be the problem you have?
Just noticed, when you click the 4 link click on accessories.