If a locked column is hidden, its width is still added to the total locked width. Simple test case:

Code:
/*

This file is part of Ext JS 4

Copyright (c) 2011 Sencha Inc

Contact:  http://www.sencha.com/contact

Commercial Usage
Licensees holding valid commercial licenses may use this file in accordance with the Commercial Software License Agreement provided with the Software or, alternatively, in accordance with the terms contained in a written agreement between you and Sencha.

If you are unsure which license is appropriate for your use, please contact the sales department at http://www.sencha.com/contact.

*/
Ext.require([
    'Ext.grid.*',
    'Ext.data.*',
    'Ext.util.*',
    'Ext.state.*'
]);

Ext.onReady(function() {
    Ext.QuickTips.init();

    // sample static data for the store
    var myData = [
        ['3m Co',                               71.72, 0.02,  0.03,  '9/1 12:00am'],
        ['Alcoa Inc',                           29.01, 0.42,  1.47,  '9/1 12:00am'],
        ['Altria Group Inc',                    83.81, 0.28,  0.34,  '9/1 12:00am'],
        ['American Express Company',            52.55, 0.01,  0.02,  '9/1 12:00am'],
        ['American International Group, Inc.',  64.13, 0.31,  0.49,  '9/1 12:00am'],
        ['AT&T Inc.',                           31.61, -0.48, -1.54, '9/1 12:00am'],
        ['Boeing Co.',                          75.43, 0.53,  0.71,  '9/1 12:00am'],
        ['Caterpillar Inc.',                    67.27, 0.92,  1.39,  '9/1 12:00am'],
        ['Citigroup, Inc.',                     49.37, 0.02,  0.04,  '9/1 12:00am'],
        ['E.I. du Pont de Nemours and Company', 40.48, 0.51,  1.28,  '9/1 12:00am'],
        ['Exxon Mobil Corp',                    68.1,  -0.43, -0.64, '9/1 12:00am'],
        ['General Electric Company',            34.14, -0.08, -0.23, '9/1 12:00am'],
        ['General Motors Corporation',          30.27, 1.09,  3.74,  '9/1 12:00am'],
        ['Hewlett-Packard Co.',                 36.53, -0.03, -0.08, '9/1 12:00am'],
        ['Honeywell Intl Inc',                  38.77, 0.05,  0.13,  '9/1 12:00am'],
        ['Intel Corporation',                   19.88, 0.31,  1.58,  '9/1 12:00am'],
        ['International Business Machines',     81.41, 0.44,  0.54,  '9/1 12:00am'],
        ['Johnson & Johnson',                   64.72, 0.06,  0.09,  '9/1 12:00am'],
        ['JP Morgan & Chase & Co',              45.73, 0.07,  0.15,  '9/1 12:00am'],
        ['McDonald\'s Corporation',             36.76, 0.86,  2.40,  '9/1 12:00am'],
        ['Merck & Co., Inc.',                   40.96, 0.41,  1.01,  '9/1 12:00am'],
        ['Microsoft Corporation',               25.84, 0.14,  0.54,  '9/1 12:00am'],
        ['Pfizer Inc',                          27.96, 0.4,   1.45,  '9/1 12:00am'],
        ['The Coca-Cola Company',               45.07, 0.26,  0.58,  '9/1 12:00am'],
        ['The Home Depot, Inc.',                34.64, 0.35,  1.02,  '9/1 12:00am'],
        ['The Procter & Gamble Company',        61.91, 0.01,  0.02,  '9/1 12:00am'],
        ['United Technologies Corporation',     63.26, 0.55,  0.88,  '9/1 12:00am'],
        ['Verizon Communications',              35.57, 0.39,  1.11,  '9/1 12:00am'],
        ['Wal-Mart Stores, Inc.',               45.45, 0.73,  1.63,  '9/1 12:00am']
    ];

    /**
     * Custom function used for column renderer
     * @param {Object} val
     */
    function change(val) {
        if (val > 0) {
            return '<span style="color:green;">' + val + '</span>';
        } else if (val < 0) {
            return '<span style="color:red;">' + val + '</span>';
        }
        return val;
    }

    /**
     * Custom function used for column renderer
     * @param {Object} val
     */
    function pctChange(val) {
        if (val > 0) {
            return '<span style="color:green;">' + val + '%</span>';
        } else if (val < 0) {
            return '<span style="color:red;">' + val + '%</span>';
        }
        return val;
    }

    // create the data store
    var store = Ext.create('Ext.data.ArrayStore', {
        fields: [
           {name: 'company'},
           {name: 'price',      type: 'float'},
           {name: 'change',     type: 'float'},
           {name: 'pctChange',  type: 'float'},
           {name: 'lastChange', type: 'date', dateFormat: 'n/j h:ia'}
        ],
        data: myData
    });

    // create the Grid
    var grid = Ext.create('Ext.grid.Panel', {
        store: store,
        columnLines: true,
        columns: [{
            text     : 'Company Name',
            locked   : true,
            width    : 200,
            sortable : false,
            dataIndex: 'company'
        },{
            text     : 'Price',
            locked   : true,
            width    : 125,
            hidden : true,
            renderer : 'usMoney',
            dataIndex: 'price'
        },{
            text     : 'Change',
            width    : 125,
            sortable : true,
            renderer : change,
            dataIndex: 'change'
        },{
            text     : '% Change',
            width    : 125,
            sortable : true,
            renderer : pctChange,
            dataIndex: 'pctChange'
        },{
            text     : 'Last Updated',
            width    : 135,
            sortable : true,
            renderer : Ext.util.Format.dateRenderer('m/d/Y'),
            dataIndex: 'lastChange'
        }],
        height: 350,
        width: 600,
        title: 'Locking Grid Column',
        renderTo: 'grid-example',
        viewConfig: {
            stripeRows: true
        }
    });
});