Thank you for clarifying that point. So if I want all 4 RCA outputs to be line level, then I will use the monitor volume control to adjust outputs 1 and 2 to match 3 and 4 in amplitude. This can be done by viewing on an oscilloscope.
I found a copy of the 2i4 manual online, but being a non-musician the logic of having the RCA outputs separately controlled might have eluded me. All the more confusing due to the 1-2, 3-4 switch to the headphone output allowing for volume adjustment of either two sets of channels at the phones.
Anyway, I think I have a grip on it now.