How to sort csv by specific column

I am trying to sort a csv containing temperatures to the 4th column.

Sort -n -k4 temperature.csv

      

        
        
        
      

    

As a result, I get the following:

2017-06-24 11:20,23.57,19.0,16.7,0.087,3.615
2017-06-24 11:25,23.51,19.0,16.7,0.087,3.689
2017-06-24 12:45,22.03,19.0,17.1,0.096,4.152
2017-06-24 13:00,21.92,19.0,17.1,0.096,4.229
2017-06-24 14:00,22.22,19.0,17.4,0.197,4.639
2017-06-24 14:25,22.21,19.0,17.5,0.197,4.774
2017-06-24 15:10,22.30,19.0,17.1,0.134,5.472
2017-06-24 16:00,22.42,19.0,17.3,0.134,5.93
2017-06-24 17:45,22.07,21.0,17.0,0.144,6.472
2017-06-24 18:25,21.90,21.0,16.9,0.15,6.814
2017-06-24 19:40,23.01,21.0,16.9,0.318,8.503

      

        
        
        
      

    

As you can see, the 4th column is not sorted correctly. I would expect 17.5 on the first line and 16.7 on the last line.

I also tried this:

sort -n -t. -k4,1n temperature.csv

      

        
        
        
      

    

The result is exactly the same as in the previous example. Can anyone give me a hint?