Before I start writing code for things, I wanted to make sure there is nothing wrong with it.
I have an Arduino Ethernet which I'm using pretty much all the available I/O pins on. I stayed away from digital pins 0,1,4,10-13 because they have special purposes. I am using analog pins 0-5 and digital pins 2,3,5-9. In other words, everything BUT what I know has a special function. I now need 1 more pin... The Arduino Ethernet has an SD card it can read/write to. I'm not using it nor do I plan on using it. Therefore, can I use pin 4 as another I/O? My logic is yes. If I'm not using it, then it shouldn't matter.
An onboard microSD card reader, which can be used to store files for serving over the network, is accessible through the SD Library. Pin 10 is reserved for the Wiznet interface, SS for the SD card is on Pin 4.
I'm using the Servo, Ethernet, Wire, EPPROM, SPI, and Serial libraries.