This fix works for me since 4.0.1

Code:
Ext.override(Ext.grid.Lockable, {
   reconfigureLockable: function(store, columns) {
      var me = this,
          lockedGrid = me.lockedGrid,
          normalGrid = me.normalGrid;


      if (columns) {
          Ext.suspendLayouts();
          columns = me.processColumns(columns);
          lockedGrid.setWidth(columns.lockedWidth);
          lockedGrid.reconfigure(store, columns.locked.items);
          normalGrid.reconfigure(store, columns.normal.items);
          Ext.resumeLayouts(true);
      }


      if (store) {
          store = Ext.data.StoreManager.lookup(store);
          me.store = store;
          lockedGrid.bindStore(store);
          normalGrid.bindStore(store);
      } else {
          lockedGrid.getView().refresh();
          normalGrid.getView().refresh();
      }
   }
});