I've looked into it and it's tough to multiplex the LED's or buttons.
I have 4 buttons and 4 LED's connected to pins 2-9, most multiplexers I've found so far turn 4 pins into 8 and I need 4 pins for each anyway.
if that doesn't work for some reason I'll buy a shift register and use it for the buttons/LED's.
If only one of your 4 leds is ever lit at once, this can be done with 3 Arduino outputs, saving a pin, using a technique called Charlieplexing. Sounds slightly scary, but not too difficult for only a small number of leds, and an interesting thing to learn about.