Not sure if the intention was to no longer support this attribute but it doesn't appear to work in 4.

HTML Code:
new Ext.form.ComboBox({
    id:'kpi_list_combo',
    store: METRICS.kpi_list_grid_store,        
    width:300,    
    listWidth:600,    
    emptyText:'Key Performance Indicator',
    valueField: 'kpi_id'
})