Set tab width for less output

Use:

git diff --color=always | less -r

--color=always is there to tell git to output color codes even if the output is a pipe (not a tty). And -r is there to tell less to interpret those color codes and other escape sequences. Use -R for ANSI color codes only.

The curses program tabs will allow you to change what the terminal believes to be the width of a ^I. This would make a simple script

tabs -4
cat "$@"
tabs -8

However, the processing of tab characters on terminals is notoriously wonky and I'm of the impression that you should never mess with them. I suggest using expand as in:

expand -4 "$@"

which is actually closer to what you intend.

added in reply to comment:

Far too many scripts count on cat meaning /bin/cat which explicitly does not change tabs. I'm not sure if you mean to replace or supersede /bin/cat, but you shouldn't. Better would be:

alias tcat='expand -4'

or

function tcat() {
    expand -4 "$@"
}

or similar.