W4 has much difference between W3. I was having lot of different audio devices, but T51+W3 sounds amazing! This is the best friend for them.
I've tried Clip+, iPod5.5G Rockbox, iClassic 2010, Touch 3 and 4G, Cowon D2,C2,J3, Zune HD, HFM601, iPad2, Galaxy Tab, Galaxy Player G50 and Nokias:)
W4 sounds great on iDevices. True China Players sounds bad with W4.
W4 has little plastic in their sound. If someone likes popular Vsonic GR07 Westone 4 will be good upgrade, they are similar.
I don't find any upgrade of Westone 3 and i think that don't need it.