Change gallery shortcode default columns ( WordPress 4.1 as of writing )

changeGalleryShortcodeDefaultColumns

<?php
/**
 * Custom default column setting for gallery shortcode.
 * 
 * Original galleryDefaults settings is in wp-includes/js/media-editor.js
 * Need this setting for admin panel
 *
 */


add_action('print_media_templates', function(){
  ?>
  <script>
    jQuery(document).ready(function(){
      // Original galleryDefaults settings is in wp-includes/js/media-editor.js
      // Need this setting for admin panel
      _.extend(wp.media.galleryDefaults, {
        columns: '4'
      });
    });

  </script>
  <?php

});




/**
 * Custom gallery shortcode
 * Original gallery shortcode is in wp-includes/media.php
 *
 */

add_filter("post_gallery", "my_gallery",10,2);
function my_gallery($output, $attr) {
    global $post;


	static $instance = 0;
	$instance++;

	if ( ! empty( $attr['ids'] ) ) {
		// 'ids' is explicitly ordered, unless you specify otherwise.
		if ( empty( $attr['orderby'] ) ) {
			$attr['orderby'] = 'post__in';
		}
		$attr['include'] = $attr['ids'];
	}

/*
  // Removed for preventing endless loop
	$output = apply_filters( 'post_gallery', '', $attr );
	if ( $output != '' ) {
		return $output;
	}
*/

	$html5 = current_theme_supports( 'html5', 'gallery' );
	$atts = shortcode_atts( array(
		'order'      => 'ASC',
		'orderby'    => 'menu_order ID',
		'id'         => $post ? $post->ID : 0,
		'itemtag'    => $html5 ? 'figure'     : 'dl',
		'icontag'    => $html5 ? 'div'        : 'dt',
		'captiontag' => $html5 ? 'figcaption' : 'dd',
		'columns'    => 4,
		'size'       => 'thumbnail',
		'include'    => '',
		'exclude'    => '',
		'link'       => ''
	), $attr, 'gallery' );

	$id = intval( $atts['id'] );

	if ( ! empty( $atts['include'] ) ) {
		$_attachments = get_posts( array( 'include' => $atts['include'], 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $atts['order'], 'orderby' => $atts['orderby'] ) );

		$attachments = array();
		foreach ( $_attachments as $key => $val ) {
			$attachments[$val->ID] = $_attachments[$key];
		}
	} elseif ( ! empty( $atts['exclude'] ) ) {
		$attachments = get_children( array( 'post_parent' => $id, 'exclude' => $atts['exclude'], 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $atts['order'], 'orderby' => $atts['orderby'] ) );
	} else {
		$attachments = get_children( array( 'post_parent' => $id, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $atts['order'], 'orderby' => $atts['orderby'] ) );
	}

	if ( empty( $attachments ) ) {
		return '';
	}

	if ( is_feed() ) {
		$output = "\n";
		foreach ( $attachments as $att_id => $attachment ) {
			$output .= wp_get_attachment_link( $att_id, $atts['size'], true ) . "\n";
		}
		return $output;
	}

	$itemtag = tag_escape( $atts['itemtag'] );
	$captiontag = tag_escape( $atts['captiontag'] );
	$icontag = tag_escape( $atts['icontag'] );
	$valid_tags = wp_kses_allowed_html( 'post' );
	if ( ! isset( $valid_tags[ $itemtag ] ) ) {
		$itemtag = 'dl';
	}
	if ( ! isset( $valid_tags[ $captiontag ] ) ) {
		$captiontag = 'dd';
	}
	if ( ! isset( $valid_tags[ $icontag ] ) ) {
		$icontag = 'dt';
	}

	$columns = intval( $atts['columns'] );
	$itemwidth = $columns > 0 ? floor(100/$columns) : 100;
	$float = is_rtl() ? 'right' : 'left';

	$selector = "gallery-{$instance}";

	$gallery_style = '';

	/**
	 * Filter whether to print default gallery styles.
	 *
	 * @since 3.1.0
	 *
	 * @param bool $print Whether to print default gallery styles.
	 *                    Defaults to false if the theme supports HTML5 galleries.
	 *                    Otherwise, defaults to true.
	 */
	if ( apply_filters( 'use_default_gallery_style', ! $html5 ) ) {
		$gallery_style = "
		<style type='text/css'>
			#{$selector} {
				margin: auto;
			}
			#{$selector} .gallery-item {
				float: {$float};
				margin-top: 10px;
				text-align: center;
				width: {$itemwidth}%;
			}
			#{$selector} img {
				border: 2px solid #cfcfcf;
			}
			#{$selector} .gallery-caption {
				margin-left: 0;
			}
			/* see gallery_shortcode() in wp-includes/media.php */
		</style>\n\t\t";
	}

	$size_class = sanitize_html_class( $atts['size'] );
	$gallery_div = "<div id='$selector' class='gallery galleryid-{$id} gallery-columns-{$columns} gallery-size-{$size_class}'>";

	/**
	 * Filter the default gallery shortcode CSS styles.
	 *
	 * @since 2.5.0
	 *
	 * @param string $gallery_style Default CSS styles and opening HTML div container
	 *                              for the gallery shortcode output.
	 */
	$output = apply_filters( 'gallery_style', $gallery_style . $gallery_div );

	$i = 0;
	foreach ( $attachments as $id => $attachment ) {

		$attr = ( trim( $attachment->post_excerpt ) ) ? array( 'aria-describedby' => "$selector-$id" ) : '';
		if ( ! empty( $atts['link'] ) && 'file' === $atts['link'] ) {
			$image_output = wp_get_attachment_link( $id, $atts['size'], false, false, false, $attr );
		} elseif ( ! empty( $atts['link'] ) && 'none' === $atts['link'] ) {
			$image_output = wp_get_attachment_image( $id, $atts['size'], false, $attr );
		} else {
			$image_output = wp_get_attachment_link( $id, $atts['size'], true, false, false, $attr );
		}
		$image_meta  = wp_get_attachment_metadata( $id );

		$orientation = '';
		if ( isset( $image_meta['height'], $image_meta['width'] ) ) {
			$orientation = ( $image_meta['height'] > $image_meta['width'] ) ? 'portrait' : 'landscape';
		}
		$output .= "<{$itemtag} class='gallery-item'>";
		$output .= "
			<{$icontag} class='gallery-icon {$orientation}'>
				$image_output
			</{$icontag}>";
		if ( $captiontag && trim($attachment->post_excerpt) ) {
			$output .= "
				<{$captiontag} class='wp-caption-text gallery-caption' id='$selector-$id'>
				" . wptexturize($attachment->post_excerpt) . "
				</{$captiontag}>";
		}
		$output .= "</{$itemtag}>";
		if ( ! $html5 && $columns > 0 && ++$i % $columns == 0 ) {
			$output .= '<br style="clear: both" />';
		}
	}

	if ( ! $html5 && $columns > 0 && $i % $columns !== 0 ) {
		$output .= "
			<br style='clear: both' />";
	}

	$output .= "
		</div>\n";

	return $output;
}