The SPI chip select is ACTIVE LOW, thus this:
// disable SD SPI
pinMode(4,OUTPUT);
digitlWrite (4,HIGH);
// disable w5100 SPI
pinMode(10,OUTPUT);
digitalWrite(10,HIGH);
// rest of setup code
DISABLES these two spi devices so the the individual code for each of them can run without conflict.
This statement doesn't seem to make sense , first because "open" does not denote any logical state. (such as HIGH or LOW)
"open" is most commonly used in electronics to denote "no connection" , hence it has no place in this discussion.
Additionally, the following statement seems to imply the chip select is active HIGH, which it is not.
See attached datasheet for a SPI device (PINOUT)
Pin 10 Has to be open for the SD card to work Hence why it is High and not used again!
and following link for correct active state for SPI chip select (LOW)
MCP4162-102-P.pdf (2.21 MB)