Hi George,

Adding new column to the mainbody seems much work to do.
You should check those mainbody file (mainbody.php, mainbody-content-left.php, mainbody-content-right.php) and those files in tpls/blocks/mainbody folder.

The idea is simple.
You can add new file four-columns.php file with the content

<?php

/**

 * @package   T3 Blank

 * @copyright Copyright (C) 2005 - 2012 Open Source Matters, Inc. All rights reserved.

 * @license   GNU General Public License version 2 or later; see LICENSE.txt

 */



defined('_JEXEC') or die;



/**

 * Mainbody 3 columns, content in center: sidebar1 - content - sidebar2

 */

?>



<div id="t3-mainbody" class="container t3-mainbody">

	<div class="row">



		<!-- MAIN CONTENT -->

		<div id="t3-content" class="t3-content col-xs-12 col-md-6  col-md-push-2">

			<?php if($this->hasMessage()) : ?>

			<jdoc:include type="message" />

			<?php endif ?>

			<jdoc:include type="component" />

		</div>

		<!-- //MAIN CONTENT -->



		<!-- SIDEBAR 1 -->

		<div class="t3-sidebar t3-sidebar-1 col-xs-4 col-md-2 col-md-pull-6 <?php $this->_c($vars['sidebar1']) ?>">

			<jdoc:include type="modules" name="<?php $this->_p($vars['sidebar1']) ?>" style="T3Xhtml" />

		</div>

		<!-- //SIDEBAR 1 -->

	

		<!-- SIDEBAR 2 -->

		<div class="t3-sidebar t3-sidebar-2 col-xs-4 col-md-2 <?php $this->_c($vars['sidebar2']) ?>">

			<jdoc:include type="modules" name="<?php $this->_p($vars['sidebar2']) ?>" style="T3Xhtml" />

		</div>

		<!-- //SIDEBAR 2 -->



		<!-- ADVERTISING -->

		<div class="t3-sidebar t3-sidebar-3 col-xs-4 col-md-2 <?php $this->_c($vars['adspos1']) ?>">

			<jdoc:include type="modules" name="<?php $this->_p($vars['adspos1']) ?>" style="T3Xhtml" />

		</div>

		<!-- //ADVERTISING -->

	

	</div>

</div>

Then you should check for condition to load this file in those tpls/blocks/[mainbody].php file.

<?php
/**
* @package T3 Blank
* @copyright Copyright (C) 2005 - 2012 Open Source Matters, Inc. All rights reserved.
* @license GNU General Public License version 2 or later; see LICENSE.txt
*/

defined('_JEXEC') or die;
?>

<?php



/**

 * Mainbody 3 columns, content in center: sidebar1 - content - sidebar2

 */



// positions configuration

$sidebar1 = 'sidebar-1';

$sidebar2 = 'sidebar-2';

$adspos1 = 'adspos-1';



$sidebar1 = $this->countModules($sidebar1) ? $sidebar1 : false;

$sidebar2 = $this->countModules($sidebar2) ? $sidebar2 : false;

$adspos1 = $this->countModules($adspos1) ? $adspos1  : false;



// detect layout

if ($sidebar1 && $sidebar2 && $adspos1) {

	$this->loadBlock('mainbody/four-column', array('sidebar1' => $sidebar1, 'sidebar2' => $sidebar2, 'adspos1' => $adspos1));

} else if ($sidebar1 && $sidebar2) {

	$this->loadBlock('mainbody/two-sidebar', array('sidebar1' => $sidebar1, 'sidebar2' => $sidebar2));

} elseif ($sidebar1) {

	$this->loadBlock('mainbody/one-sidebar-left', array('sidebar' => $sidebar1));

} elseif ($sidebar2) {

	$this->loadBlock('mainbody/one-sidebar-right', array('sidebar' => $sidebar2));

} else {

	$this->loadBlock('mainbody/no-sidebar');

}

You should adjust those value and update the if else condition for your need.

Regards