We use two Zoom H4, sometimes simultaneously for enregisterements 4 channels (surround).
It is true that there is background noise on the recording appears to be related to the sampling frequency or the hard disk access. Moreover, the tone of the noise (the frequency) changes with the sampling frequency (44.1 and 48 kHz.) Unfortunately, this noise also appears on the XLR (slightly lower if the sensitivity of these external microphones is supperior to the microphones embedded).
But according to my tests (and my Zoom H4), this noise does not appear if the sample rate is set to 96 kHz!
If a signal / noise ratio maximum is required, you can save and convert 96 kHz to 48 kHz with Wavelab or other comparable software if you do not need a range of up to 40 kHz bandwidth (which is rarely case because the microphones "end" usually 16 or 20 kHz).
In this way we can overcome this shortcoming and enjoy the quality of this little recorder that is still currently the most practical solution for recording anywhere and anytime. (Warning: Zoom H2 comes ...)
In any case, the price / quality ratio is excellent.