This is very strange: the 4v output will be at 1v level when set to -6db (approximately - 3dB down is a halving of the output level, voltage wise). A 4v output means that the peak voltage is 4v - there's nothing magic about 4v when compared to 1v, just that the signal is 4x as large.

From what you've said it sounds more like a grounding problem - did you try the grounding leads?

Hugo