OK.. I don't have a ton of experience with headphone fidelity (speakers are another story). I've never owned a high end pair of headphones and I'm moving to the 4vi from a modded set of JVC Marshmallows (soldered them onto the stock iPhone headphone cord).
The detail is quite a bit better. I get the "picking up new sounds in familiar recordings" thing often. The bass seems a bit punchier, but there isn't that much more of it. There isn't a *ton* of it at all, to be honest, but it *sounds* great. Very controlled. Unfortunately I can't compare this to the 3 or 5, as I haven't tried either.
The control button is kind of a pain in the ass. The stock iPhone headphones' button is a semi-flexible blob that can be pressed from any direction quite easily. The 4vi button is in a hard plastic socket with the button on one side covered by inflexible rubber. It takes a little getting used to, but after a few days I'm able to double-click to skip songs without accidentally pausing it instead. The rubber covering does start to come out, as others have mentioned, but it's easy to put back in.
Comfort and fit are reasonably good, but I find it a little more difficult to get a good fit with the 4vi than the marshmallow, but I admit this is my first interaction with silicone tips.
Anyway, if you have any questions I'd be more than happy to try to answer them.
Edit: Forgot value rating. For what I paid ($80 with expedited shipping), I find them to be a great value. I tried a friend's set of Bose Triport IE's ($100) and found them disgusting to listen to. Seriously. Awful. For $150 the 4vi's value quotient definitely nose-dives, as you can still get the Super.Fi 5 pro's for $140 at Amazon, and I believe those are generally considered to be vastly superior.