In your sidebar array as an example:
register_sidebar(array(
'id' => 'footer',
'name' => 'Footer',
'before_widget' => '<div id="%1$s" class="widget col-sm-4 %2$s">',
'after_widget' => '</div>',
'before_title' => '<h4 class="widgettitle">',
'after_title' => '</h4>',
));
Don’t worry about wrapping the sidebar in a row. Let this do it for you. See, the col-sm-4 (or whatever size, with 4) would make every widget in the Footer have a width of 4 columns.
You would need to provide more theme code for us to provide a more descriptive answer, as your theme setup and inter-dependencies might be an additional cause for error.