I've owned the W4 & GR07 (original & MKII).. I'd take the GR07 over the W4.. any day of the week. The W4 has better dynamics, soundstage depth, and imaging.. but I find the GR07 textures better, is more linear, and flat out sounds better. The W4's massive midbass boost ruins what could be quite a special phone (in my eyes). The W4 is sounded too too warm and, frankly, bloated to my ears.. in comparison to the GR07. Neither is a "perfect" phone.. but I think the GR07 does "less wrong."
As far as an upgrade to the GR07.. that's really tough.. I haven't heard anything that sounds quite like it enough to consider calling said phone a direct upgrade. The AS/G-2 is a good suggestion (I have the AS-2).. and many of the same things I love about the GR07, I love from the AS-2. It's smooth, slightly warm, with excellent bass and airy, detailed, but relaxed treble. The AS=2's midrange is terrifically linear and portrays vocals with precision. It's also on the dry side (like the GR07). Aside from that, I'm not sure. There's been some rumors (mentioned on this very thread earlier) that the GR07 MKIII is getting released in October 2013.. but at this point, it's all speculation.