The chip select for the Ethernet chip W5100 is hard wired, so you better avoid using pin 10.
The chip select for the SD card is only connected to a input of a logic gate (before going to the SD card), so if you have no SD card installed you can use pin 4.
Almost every library is able to change the pin for the chip select. So I agree with spycatcher2k, the best option is to change the library.