Slaving multiple Arduinos via SPI
the SCK lines are tied to ground via a 0.1uF capacitor (to smooth the clock signal)


Get rid of that for a start.


  The other three slaves are floating their MISO lines (third state)


That's your problem. They should all be outputs. From the datasheet:


  When the SPI is configured as a Slave, the Slave Select (SS) pin is always input. When SS is held low, the SPI is activated, and MISO becomes an output if configured so by the user. All other pins are inputs. When SS is driven high, all pins are inputs, and the SPI is passive, which means that it will not receive incoming data. Note that the SPI logic will be reset once the SS pin is driven high.


As you can see from that, you should configure all the MISO pins as outputs. However the SPI hardware will turn them into inputs if SS is high. Thus only one slave (the one you set MISO as an output) is capable of responding at present.

You should probably set them as outputs after turning on SPI slave mode. eg.

  // turn on SPI in slave mode
  SPCR |= bit (SPE);

  // have to send on master in, *slave out*
  pinMode (MISO, OUTPUT);