Force the series 4 boxes to call home and if needed, reboot.
What version Tivo software is on the s2? Have it also connect and restart it.
Sometimes you have to do a little dance of disabling, restarting, connecting, waiting, etc with the sharing settings. I haven't done that dance in awhile, but you should be able to find the instructions here to at tivo.con support area
As was already asked, can you the s2 see the other boxes?
-Roamio Pro/Slide Pro
-Stream (Gotta go unplug the sucker one of these days!)