@ w0lverine: I am admittedly a tad OCD when it comes to hissing. The rational part of me knows full well that once the music starts playing, the hiss is essentially entirely covered up, but somehow I have convinced myself that it is still there in the background, detrimentally affecting my enjoyment of the music... I know it's silly, but this is the sort of pedantry that sets us audiophiles apart from sane and practical people, right?
As for the W4, I have a bit of a love-hate relationship with it. I find the fit to be less than ideal, mainly because of the short nozzle on the W# line of IEMs. Different tips affect the sound signature drastically. Unlike me, you have custom silicon tips, and the W4 might sound great with them... but you better check with someone else that the custom tips can be used inter-changeably between the W# line and the UM# line. They appear to have slightly different shaped housings.
YMMV, but going from the UM2 to the W4, I would say expect: a bigger sound stage; less "warmth"; less bass; more treble and "sparkle" up top, better instrumental separation, and possibly worse isolation.
@Jeremypsp: We are not exaggerating! The UM2 is ridiculously loud! For instance, they are practically unusable with some digital volume controls if the increments are not small enough. For instance, with the Digizoid ZO2.3 in LOD mode, when the volume is all the way down it is too quiet, then one click up on the volume wheel and it is already too loud.