subtract the values of two columns using awk or bash

I have some text files as shown below. I would like to subtract the values of column 2 and 4 and need to create a new column to the output.

co1  co2   co3    co4

r1  15.2  13.0   21.4
r2  23    15     15.7
r3  14    8      12

desired output

co1  co2   co3   co4   diff.    

r1  15.2  13.0   21.4   -6.2
r2  23    15     15.7   7.3
r3  14    8      12     2