It occurs to me that (this not being C, after all B-P), it would be
better to just do:

if [ whatever ]; then
  mkdisk() {
    mknod $1 b $2 $(($3 * 16 + $4))
  }
  ...
else
  mkdisk() {
    mknod $1 b $2 `expr $3 * 16 + $4`
  }
  ...
fi

but you get the idea.