Subtracting numbers from adjacent columns and successive rows using awk

You can do this as below. Defer the subtraction except for the first line but get its last column value as input for the subsequent line.

awk -F'\t' 'BEGIN { OFS = FS } NR == 1 { last = $4; print;  next }{ $5 = $3 - last; last = $4  }1' file