Given a CSV file, how can we replace the values in one of the columns?

For example given this file:

examples/test_cases.csv

170,test,best,one,0.1314
171,test,best,two,0.1695
172,test,best,two,0.1784
173,test,best,three,0.3748

how can we replace the 4th column to contain the value 'CIS' in every line?