I really like #4 on you too! There seems to be a sharper/straighter/more vertical "v" in the this neckline (compared to the others), which does create definition. It's not lower cut or anything, it's just that the "v" in #4 is more vertical, and the others have a wider-looking, more open neckline. Very interesting to realize that a narrower 'v' might be better than a wider one.
As Angie said, if you were to go with #4, all you'd really need is some higher-waist underthings, and for the most part - it really looks like you'd have no trouble with a bra in this one either, as the straps are wide enough and there are lots of bras that have a deeper 'v' between the cups - no need for a specialty bra in this dress, which can be a royal pain.
Despite what you think - I think you've found 'the' dress.
I'd suggest buying #4 - hang it up, keep tags on...and you can relax. If you come across others, then fine - try them out, but #4 is pretty fabulous if you ask me. It's very elegant and glamourous-looking and a lot more flattering than you think. You look beautiful in it.
What do you think Michelle?