Filtering the_content, but still need to display the unfiltered content inside filter

First, your filter should return, not echo anything. The content will appear in the wrong place if you echo directly within the function.

The original contents of the_content are passed to the filter function as an argument-

function fds_the_content( $content ) { // <--- original value of the_content
    // append original $content
    $new_content="new content" . $content;
    return $new_content;
}

EDIT- echo $content in an output buffer-

function fds_the_content( $content ) {
    ob_start();
    ?>
    <div>
        <h4>Some Markup</h4>
        <?php echo $content; ?>
    </div>
    <?php
    return ob_get_clean();
}
add_filter('the_content', 'fds_the_content');