@Kennshin
The measured -4°C should be correct.
Reason: The reference voltage for the AD converter has been defined by yourself with 1100mV.
According to the datasheet of the sensor, the voltage which will be provided by the sensor at +125°C will be 1205mV (1205mV > VRef).
The maximum voltage which will be converted by the ADC is 1100mV. In case you will set 1205mV on the ADC pin, I would expect that there will be an overflow in the µC. Due to that the reference voltage has not been adapted to the max sensor voltage, you will get a lower temparetaure as expected.
Also the behaviour U=f(T) of the sensor is not linear according the reference points you mentioned from the datasheet.