I thought I'd post Westone's response to some questions I asked about the difference in the 3 and 4. They happen to recommend the 3 for my rock oriented music library. However I still plan on picking up the 4 versus the 3. Their description seems to mirror a lot of what you all have said.
"Hello, and thank you for you interest in our products. Sound is difficult to describe in words, but I will do my best to provide as much information as possible. The Westone 3 is a proven personal listening earphone that has a very high end audiophile sound, and I believe would work exceptionally well for you, especially for the style of music you listen to. The Westone 3 delivers a very balanced sound, and has good low end thump, without being over bearing.
The Westone 4 is really for the consummate audiophile looking for the most neutral sound in an earphone. While the specifications on the Westone 4 do show an extension in the lows, that should not be interpreted as an increase in low frequencies, but rather an extension to provide more detail. The Westone 4 has a tight frequency response, and keeps the low end response very pristine, detailed, and accurate. The mids and highs have another level of smoothness, when compared to the Westone 3, due to the speaker and cross over configuration, again to be more neutral and less colored.
The Westone 3 could be considered a fun earphone for personal listening, whereas the Westone 4 would be for the person wanting extreme detail and neutrality with as little coloration as possible from an earphone. I hope this has helped in your decision making process."