less command accepts its defaults with an environment variable
LESS, so you can
export LESS='-F -g -i -M -R -S -w -X -z-4'
at the beginning of your session.
Is it possible to change the default lines count returned by
tail in a similar fashion?
An alias is not an option, because it breaks explicit option setting (e.g. in a script):
head -n 15 -5
fails with an error in both GNU and busybox
head at least.