I received my 4.As a few days and have been enjoying myself since. I have comments and observations to add to the discussion.
1) Regarding fit, this is my first CIEM and I was really worried about a bad fit. The first time I put them on, the left one just popped in and needed no adjustment, however the right one needed a little extra wiggle to make sure it was in the right place. But once this was solved, everything felt quite comfortable, and I had no discomfort after listening for about two hours. I did have trouble getting them out, but found if I pull them out gently with a little twist, they would come right out.
2) The sound is very dependent on the recording. The first thing I listened to was Bon Iver's cover of "Can't Make You Love Me" combined with "Nick of Time". I honestly felt a little disappointed. His voice sounded the way I remember it, but the piano sounded somewhat thin. Thus the worry about missing bass began to creep in at this point.
I next went to listen to something that had a bit more bass presence, The XX. Having just seen them live, I knew what kind of sound they were shooting for. Needless to say, the 4.As put my fears to rest. Everything was handled delicately and kept in its place. The texture of the male singer's voice was there, clicks on the drum machine, the beautiful steel drums. When going back and forth between the two albums, this is where I came to the conclusion that everything is recording dependent. On the very first track I listened to, "Intro" the percussion is very prominent and a big part of the mix. However transferring over to a track like "Shelter" or "Angels" that is pretty much about vocals and the guitar/bass line, everything else is kept in check but still very much present, especially the booming bass drum hits that have the potential to take over the sound if the bass is overemphasized.
Also, Bossa Nova recordings sound amazing. I am again hooked on the Getz/Gilberto album.