co1  co2   co3    co4

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

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

NR==1{print $0, "   diff.n"}
NR>2{printf("%st%5.1fn", $0, $2-$4)}

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       2.0

awk -f so.awk data.txt

awk 'NR == 1 { $5 = "diff." } NR >= 3 { $5 = $2 - $4 } 1' <input.txt

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

awk 'BEGIN { OFS = "t" } NR == 1 { $5 = "diff." } NR >= 3 { $5 = $2 - $4 } 1' <input.txt