Why would I need 4 cables from a 3-lnb dish?
Because of the way D* processes the signal from the receivers to the LNB's to select what satellite and then what transponders you need to watch your channel.
D* uses two different voltages (13VDC and 18VDC) and then adds a 22kHz control tone to each voltage. The break down is as follows:
13VDC, no control tone = Sat 101, odd transponders
18VDC, no control tone = Sat 101, even transponders
13VDC, 22kHz control tone = Sats 110/119, odd transponders
18VDC, 22kHz control tone = Sats 110/119, even transponders
If you are using 4 tuners or less, the built in multiswitch in the LNB can handle this function, so you can run a cable from the LNB to the receiver. With more than 4 tuners in play, the 4 outputs from the LNB need to feed a multiswitch that can send the signals out to 5+ tuners.