set tab width for less output

Yes, it is possible with less -x or less --tabs, e.g. less -x4 will set the tabwidth to 4.

You can configure defaults with the LESS environment variable, e.g. LESS="-x4".

The less-specific answer has already been given, but a generic solution (for any pager) is to expand the tabs (with space characters) with the expand command before feeding to the pager:

expand -t4 file | pager

Instead of setting the tab width in command line (before you open less). You can also set the tab width within less by typing -x4