There's no such thing as "4-space hard tabs". A hard tab is just a character.
It is editor settings that decide how wide a tab should be displayed. You can set it to be two, four, eight spaces wide, and it will not alter the source code.
The matter is that SASS does support tabs for indentation, with one tab meaning one level of indentation. So there should be no problem for you.
If you're talking about the indentation of CSS files, i really don't understand why this is of any significance for your team. If you're using SASS, CSS files are not supposed to be edited and are rarely ever opened (most debugging takes place in developer tools like FireBug, are you aware of debug_info and source maps?). In production CSS is minified and all white space is purged.
Consider adopting a task runner tool like Grunt. You can leverage it to parse resulting CSS files at certain stage of the development routine cycle. You can also make use of the brilliant autoprefixer tool if you go this way.
PS Two spaces is the standard indentation for the Ruby community, which SASS is mostly a part of. You team is kinda stemming the tide.