Hi, I have IM04, and have tested IM03.
They are very different and IM04 is not really an upgrade from IM03.
IM04 has thick and smooth sound with lot of detail. A little bass heavy and some people call it "dark" but it is very good for long hours of listening.
IM03 on the other hand is more ordinary type. It gives more sparkle in treble and the sound has less thickness.
This may easily lead to impression like "oh this one has wider sound stage as well as more detailed sound !" and in fact many people claim IM03 is just superior.
I personally think that is not true and the more I listen to IM04, I can realize the sound is also very detailed.
(Can be that actually IM04 is more detailed with a help of an extra BA unit)
So it is really up to your preference, but I can say both are at outstanding level as universal IEM - enough to compete against Westones or Shures.
If you have UM Mage and SE425, may be IM03 has closer nature.
In other words if you are looking for something different, IM04 should be the way to go... but in that case be aware that you can be surprised by its "dark" sound sig at first.
Edited by tyojpn - 3/8/14 at 7:01pm