When you go to the audiologist, make sure he / she has experience with doing ear impressions for CIEMs. The audiologists where I live all mostly suck, and I had some very bad experiences as a result.

Insist that they look at your instruction sheet provided by the manufacturer who will be making your customs. Make them actually LOOK at it. I've heard so many stories about---and experienced for myself---audiologists not bothering to look at the instructions and snidely telling customers they "know what they're doing" because they've worked on hearing aids. Hearing aids aren't the same deal, no matter how much they may insist! Healthcare professions can be very smug and dismissive and convinced that their way is best, refusing to listen. Also don't let them pass you off to less skilled trainees.

The last audiologist I went to ignored the instructions and cast the impressions. When she screwed them up, she tried to put the blame on me, claiming to her boss that I never mentioned the instructions which was a blatant lie considering I repeatedly asked her to look at them. The boss himself was a complete lunatic, and ranted and raved about the instructions because they insisted cotton tips be used instead of foam. He was convinced there was no difference and was infuriated that he was put out by having to track some down. He also ranted about how expensive the customs were and how the company, located in China, was trying to "make money off of [his] work!" and how you can't trust the Chinese.

Obviously most audiologists wont be that patently insane, and that's really the most extreme of my experiences, but I cite it as a warning to make sure you speak up for yourself and don't let them get away with doing a half-assed job. I ended up having to pay for BOTH the first mistake impressions and the second correct impressions. Cost me $140 for the two. I could have contested it, but I was upset over the experience and wanted to get out as quickly as possible. You'd certainly be well within your rights to demand a do-over or refund as a result of their refusal to listen however. Just play it safe and make sure they know about CIEMs and have at least some experience with them and not just hearing aids. Even then, make sure they look at those damn instructions!

Getting your impressions returned or having to get a refit sucks and the shipping gets quite costly, so make sure you do everything you can to get the best possible impressions before hand as LFF says.

The C4 - it's explosively good.