Not enough digital outputs for SPI slave select lines (CS#)
use a 4 to 16 decoder.  you could drive the logic from an extra io pin to get 8 instead of 4. 

update:  Actually, I would probably use a SPI based shift-register to just do the CS selection, and then MUX that with the control line.  You are extremely pin limited, so you have SCLK/MOSI/CS.   You could use that last output pin to serially select which "CS" logic you will use when the actual CS goes low.