I bought a Zoom H-4 for recording my bands practice sessions, but I
found out that it could serve as a soundcard too, nice :) but there is a
problem, when i connect there is some constant high humming background
noise :) Has anyone experience with the recorders soundcard setup ? Or
is it something due to the USB ?

I have a Zoom H4 also. I have not noticed any noise other than if i have the microphone set with med or high gain.. it stays on low. From your email, I cannot tell if the noise appears when you are using the H4 as an input and another sound card for the output or if the noise only appears when the H4 is used for output. I normally use the internal soundcard for output with the H4 only as input. You might also try setting the H4 to use the two balanced jacks (instead of the microphone) and see if the noise is still around.

Matt