Add tag counts as classname in the $range of 1 to 4 depending of the post count in each category

tag-cloud-count-color.php

<?php

add_filter( 'wp_generate_tag_cloud', 'maybe_color_tag_cloud', 10, 3 );

function maybe_color_tag_cloud( $cloud, $tags, $args ) {

	$a = array();
	$counts = array();
	$range = 4;

	// Get term counts for our math needs
	foreach ( $tags as $tag )
		$counts[ $tag->term_id ] = $tag->count;

	$count_min = min( $counts ); 
	$count_unit = ( max( $counts ) - min( $counts ) ) / $range;
	$font_unit = ( max( $counts ) - min( $counts ) ) / ( $args['largest'] - $args['smallest'] );

	foreach ( $tags as $key => $tag ) {
		$a[] = sprintf( 
				'<a href="%s" class="tag-link-%d tag-style-%s" style="font-size:%s%s;">%s</a>',
				esc_url( $tag->link ),
				$tag->term_id,
				max( 1, round( ( $tag->count - $count_min ) / $count_unit ) ),
				$args['smallest'] + ( ( $tag->count - $count_min ) / $font_unit ),
				$args['unit'],
				esc_html( $tag->name )
			);
	}

	return implode( $args['separator'], $a );

}