copy this class to your project lib
https://github.com/atk4/atk4/blob/master/lib/QuickSearch.php#L99
and change line 116
$or->where($field, 'like', $v);
You can change name on new class and use them both - original and new. To define which QuichSearch you want to use set class name:
$grid->addQuickSearch($fields,'NewQuickSearch');
https://github.com/atk4/atk4/blob/master/lib/Grid/Advanced.php#L129