Just spoke with Ken at PortaBrace. He recommended against using the FR2 bag for the R-4 due to their different designs. Personally, I don't see why it would matter. There's a difference of only a few millimeters in the FR2/R4 physical dimensions, both units require side flap access for various connectors (which the bag has), and both units have buttons on the top face of the unit (looks like the only way to access the top face is to slide one's hand into the main cavity - no biggie, that's what I do today).
At any rate, I'm going to hold off for a bit, because he indicated they have an R-4 bag coming out soon. He wasn't able to give me specific dates, but he said design is completely done, and PB simply needs to place the production order. Who knows how long, then - could be weeks, could be months. But he said to email him at ken@portabrace.com and he'll contact me when the R-4 bag becomes available.
Also found out the
AR-FR2 (and likely the R4 bag) come standard with the
RM-Multi pouch, but if that's too small (as is likely the case for me) it's possible to upgrade to the
RM-Deluxe. Both are detachable via velcro.