The FOH returns from the S4000 FOH box do not appear in the M-48RCS, allowing assignment to groups
By design, the M-48 units act as "Split" locations. In REAC, Split locations are "listening posts" that receive REAC ports only from the Master REAC device. In your case, that's the 3208 Stage Unit. If you need to hear console returns in your M-48 units, you will need to route the outputs of your 3208 Stage Unit directly to inputs of your 3208 Stage Unit. This will use up input ports at the Stage Unit, but it will work. If you use a Y cable for these XLR's, you can send your console returns to both the 3208 input ports and your amps/wedges.