Try "Allied Electronics".
Is it necessary that the the 4 wire signal being sent to the main room be continuous, I mean, would it
matter if the signal being sent oscillates at a rapid rate? As a last measure, maybe you could still utilize
the 74154. Have each 74154 output go to a switch, and a "common" from all switches coming back to 4
7408 AND gates, whose other inputs are the 7490/74154 binary link, then send the AND gate outputs...
555 the 7490/74154 to "sample" each switch for continuity, and when present, it will trip the ANDs (that
apply per the binary 7490/74154 link). Start the 555 at a slower rate and increase it's speed until you
develop problems, then back off the timer (maybe about 10%). If you need a long transmission line,
then use some transistors to increase the voltage sent on your four wires. One good thing about this
circuit is, you could reverse this entire process on the receiving end and indicate more than one switch
being activated (inputs to the 74154) and it would tell you all of the switches that are activated. If need
be (for a continuous signal, or if the timer has to run at a lower rate than ideal <causes your LEDs? to
flash>, buffer each 74154 receiving output with some capacitors to hold the monitor high long enough
for the next received sampled signal). Just an idear...