How to calculate the value of a current sense resistor for use with an ADC?
170mV at 4A is 0.68W. 40mV at 4A is 0.16W, saving 0.52W. Less heat means lower temperature and/or less PCB area used. The lower voltage may reduce ADC resolution, but lower temperature variation in the sense resistor improves its accuracy. 

The LTC2975 has a 16 bit ADC. Using only 40mV out of 170mV loses about 2 bits, but you still get 1mA resolution in 4A which is 0.025%. Other factors will make the absolute accuracy much worse, so the small amount you lose by not using the full ADC range is not worth worrying about. On the plus side you have 400% headroom to handle current surges and ripple.