Speeding up Girino gives strange readouts
From the ATmega328P datasheet, section 29.8, "ADC Characteristics":


  Clock Frequency — Min.: 50kHz, Max: 1000kHz


With a 16MHz system clock this means that a /16 prescaler is the most the ADC is considered reliable under for a reasonable 10 bit result (for a full 10 bit result the maximum frequency is 200kHz, which corresponds to a prescaler of /80). 

As the frequency increases the sample-and-hold circuitry of the ADC gets less time to do the "sample" part before conversion begins. As such any correctness goes out the window, certainly at the lower bits. The fact that you got any sort of result that you did at /4 is luck.

TL;DR: The ADC is too slow.