@Blackmamba: I can't speak for Poimandres, but I'm in the exact same boat as you and I think I've decided on the 4.A's. Basically, it came down to build quality and sound signature.
The Heir Audio reviews I've read, no matter who from and what model, all speak of a nearly peerless build quality. They use words like perfection, mastery, and material clarity. When compared to the big guys in the industry like JH Audio and Ultimate Ears, Heir Audio is described as being as good as if not better in terms of build quality.
In terms of SQ, the 4.A is supposed to have a more measured, intent sound where the 1964-Q is supposed to have a bit more umph at the low-end. I e-mailed the Wizard yesterday and found a response in my inbox this morning. I stated my tastes were highly eclectic but lived more on the rock/electronic/jazz side of the spectrum with the occasional foray into symphonies and hip hop. My desire was that if I heard big bass in the track it's because it was put there by the artist but that if it wasn't there then it wasn't added. Based on that, he said the 4.A was the correct CIEM out of their lineup for me and I think I'm willing to defer to his judgement.
He aslo informed me that they can be driven reasonably well from something as simple as an iPhone but that they do begin to really shine when you start adding an AMP to the line. Considering these will take over, at the least, as my commute/work headphone, that all sounds wonderful to me.
So if you sound like me then I'd say go for the 4.A. If, however, you tend to listen to music that has a lot more bass and really want to push that bass, I'd say go for the 3.A or the 1964-Q. It sounds like they have a more similar sound signature, from what I've been reading.
Also, for the rest of you who have HA 4.A's, I do believe I shall soon be joining your ranks! On a side note, what are those of you who have them already using for an AMP setup? I'm interested in something highly portable that I can easily take to and from work. If it's USB powered all the better.