I personally see the W4 as the successor to the W2. They are clearly from the same family. IMO, the W4 does bass, mids, treble and soundstage all better than the W2, while still maintaining that ability to do all genres justice, preserving that slightly warm sound and playing nicely with older recordings. The W2 is a great earphone. In my experience at its price point, I think its unmatched, though it sounds like the GR07 may beat it. I just haven't heard it yet. I think the W4 is better though than the W2. For me, W2 is the affordable option for those wanting what the W4 does well. It's the B+ version at $150 less than the A version.
I must say that for me the W2 is a little more comfortable since is smaller than the W4, though the W4 is very comfortable, too.