These are some values for parallel resistors that give you 7 outputs from 4 switches if you use combinations of two at a time.
Code:
Rgnd: 220
SW R Vo
SW1 120 0.65
SW2 270 0.45
SW3 680 0.24
SW4 1200 0.15
SW1/2 83.07 0.73
SW2/3 193.26 0.53
SW3/4 434.04 0.34
The resistors are 120, 270, 680, 1200 ohms (220 RHI to gnd). This should activate outputs 6, 4, 2 and 1 for single switch presses, and outputs 7, 5 and 3 for combination switch presses.
You may have a problem using combinations of buttons because you won't be able to press or release them at exactly the same time, and therefore you will have unwanted outputs when you try.