I don't own 4.Ai's yet but I do have them on order so I guess that makes me qualified to answer this particular question.
I've been down the customs route before with a pair JH13Pro's and it ended up being pretty much a nightmare in which I ended up wasting hundreds of dollars in shipping costs and hundreds more in losses when I finally sold them.
It started out with only being able to find one local audiologist who would do impressions, she had her own ideas about what was right but I was assure
I've been down the customs route before with a pair JH13Pro's. They were sent back at least a half dozen times due to fit issues, plus I developed a case of TMJ shortly after I got them which continues to cause pain around the bend in my left ear making the deeper insertion of most customs a no go. I finally gave up when, after six months of refits, I was told I'd have to go get new deeper impressions and start over again.
Between the losses incurred selling them, the impressions and the shipping costs for refits, I lost well over half the money I invested in them so even if I didn't have the issues with pain in my left ear I don't think I could go down that route again. I'd much rather take a potential hit in quality for the assurance that I should be able to find a tip that works and if they don't suit me I can sell them at much less of a loss than customs.