ATTiny 85 analog input pin number problem
See the analogPinToChannel function.

static const uint8_t A0 = 6;
static const uint8_t A1 = 7;
static const uint8_t A2 = 8;
static const uint8_t A3 = 9;

...

#define analogPinToChannel(p)   ( (p) &lt; 6 ? (p) : (p) - 6 )


Using A2, which is 8, will return 2 (8-6) from the analogPinToChannel function. 

Using 4 will give 4. There is no (useful) analog channel 4.