So, if I add 18dB boost/pad to Out4, both the block and hardware meters increase and start to look very similar to the Out1 meters. Without the boost/pad on Out4, the Out1 meters read consistently ~20dB lower than Out4. So, there must be an intrisic boost/pad on Out1 (which matches your "out1 has different gain staging" comments and is where my "~20dB gain" comment comes from). The Out4 meter is now reading 18dB more than I see at the DAW which makes it apparent that it is not metering the hardware output but rather the converters (as you say).
So, this is starting to make sense if I think of the meters (block and hardware) as being at the converter and Out1 having an intrinsic boost/pad.
I only really care about this because I want Out1 and Out4 to *output* the same level so I can, for example, run amp+cab+reverb -> Out1 and amp+cab->Out4 in case I want to replace the reverb later. I want these tracks to be interchangeable in the DAW, so they need to come in at the same level. And, of course, I don't want either path to clip. In this case, the Axe metering doesn't help me balance the output levels so I have to use the DAW.