The EXTJS 4.0 beta 2 API documentation is missing a description for Ext.panel.Panel.tools. The source code of initTool indicate that the tools property still exists but no documentation exists for it.

Code:
 /**
     * @private
     * Tools are a Panel-specific capabilty.
     * Panel uses initTools. Subclasses may contribute tools by implementing addTools.
     */
    initTools: function() {
        var me = this;

        me.tools = me.tools || [];    //<------- do documentation exists for me.tools

        // Add a collapse tool unless configured to not show a collapse tool
        // or to not even show a header.
        if (me.collapsible && !(me.hideCollapseTool || me.header === false)) {
            me.collapseDirection = me.collapseDirection || me.headerPosition || 'top';
            me.collapseTool = me.expandTool = me.createComponent({
                xtype: 'tool',
                type: 'collapse-' + me.collapseDirection,
                expandType: me.getOppositeDirection(me.collapseDirection),
                handler: me.toggleCollapse,
                scope: me
            });

            // Prepend collapse tool is configured to do so.
            if (me.collapseFirst) {
                me.tools.unshift(me.collapseTool);
            }
        }

        // Add subclass-specific tools.
        this.addTools();

        // Make Panel closable.
        if (this.closable) {
            this.addCls(this.baseCls + '-closable');
            this.addTool({
                type: 'close',
                handler: Ext.Function.bind(this.close, this, [])
            });
        }

        // Append collapse tool if needed.
        if (me.collapseTool && !me.collapseFirst) {
            me.tools.push(me.collapseTool);
        }
    },