Very likely this has to do with “scale mapping”. You can find the settings under “keyboard mapping” in the config menu. (keyboard mapping will be renamed to scale mapping in the next versions)
Relevant for you are the first two parameters:
This sets the point were µTune starts mapping the scales. In both up and down directions.
Middle note is set to 60 by default, mapped to 5V. This means your scale is repeated from note 60 upwards and downwards. Since your scale is very short (only 4 notes), note number 56 is mapped to 4V, 52 to 3V,… and finally note number 40 to 0V.
This means any note number below 40 will produce 0V. That is why you can not hear any change when inputting low voltages/note numbers
If you work with low input voltages between 0V and 5V, I suggest you set the Middle note and Middle voltage lower. For example 0 and 0V, or 12 and 1V.
This should do the trick for you..