Allectronics or Electronic Goldmine (maybe it was place else...) had 'Y' splitter cables, from 1/4 stereo to two RCA. Not sure about 'Boomboxes', do they have audio inputs? The switching, guess simplest would be 4 X SPDT, that way you could switch the R/L at the same time, but seems a little pointless, just send the signal out to all four, use the stereo you are listening to switch it in if you want the piped music, or something else (cd, radio, casette, 8-track...).