Note: The instructions change for 1.4.0 and later. Please see addendum below.
This is an awesome question. Yes, it's possible to "crossfade all four inputs from a single X-Y." The solution looks like this:
Make 4 Mic Knobs
Set All 4 Knobs to Be XFader Components
Make Mic Supers 2 and 4 Inverted
Make All Four Knobs Supercontrols
CREATE 4 ADDITIONAL KNOBS
These are the actual controllers that are going to control the mics.
MAKE EACH OF THE FINAL MICS INVERTED
MAKE EACH OF THE SUPERCONTROLS CONTROL ITS CORRESPONDING SUBCONTROL
NOW WE'RE READY FOR THE XY PAD
Make an XY Pad
Make its X-axis a supercontrol of Mic 1 Super and Mic 2 Super
Make its Y-axis a supercontrol of Mic 3 Super and Mic 4 Super. See this question if necessary: How to Access Y Dimension
That's it. Now the center of ech axis will make the final Mic knobs spin up one at a time. Bottom middle is Mic 3. Top middle is Mic 4. Right middle is Mic 2. Left Middle is Mic 1.
Now you can do a lot of creative things. Just messing around for a minute or two I wound up with this result:
Addendum: Changes for 1.4.0 and after, now with "Cossfader Overlap."
You'll have to postpone this instruction, "Set All 4 Knobs to Be XFader Components" until AFTER they are subcontrols of the XY pad. Otherwise you won't even see the option.
Crossfader piece is now called "Crossfader Overlap" and is a slider. Set the slider to ".5" for all four knobs. This will have them crossfade at the center.