Default WordPress to use 4 column galleries

gistfile1.php

<?php

add_filter( 'shortcode_atts_gallery', 'sheri_galleries_default_to_four_columns', 10, 3 );

function sheri_galleries_default_to_four_columns( $atts, $defaults, $raw_atts ) {

	// Don't override manually-set number of columns
	if ( ! empty( $raw_atts['columns'] ) ) {
		return $atts;
	}

	$atts['columns'] = 4;

	return $atts;
}