shiny multiple-interactive select V2

library(shiny)
df <- data.frame(
   brand=c("a","a","a","a","a","a","b","b","b","b","b","c","c","c","c","c"),
   model=c("a21","a21","a21","a21","a31","a31","b55","b55","b56","b56","b56","c33","c34","c34","c33","c34"),
   tax=c("y","x","y","x","x","y","x","x","x","y","y","x","y","x","y","y"),
   seller=c("B","B","B","C","C","D","A","D","B","C","C","B","A","C","C","B"),
   price=c(1000,1100,1500,1400,1200,1300,2000,2200,1900,1800,1700,800,900,850,950,880), stringsAsFactors = FALSE)
app = shinyApp(
  ui = shinyUI(fluidPage(
   titlePanel("dynamic renderUI test"),
   sidebarLayout(
     sidebarPanel(
        selectInput("brand",label="choice the brand",choices=c("all", sort(unique(df$brand)))),
        selectInput("model",label="choice the model",choices=c("all",sort(unique(df$model)))),
        selectInput("tax",label="choice the tax",choices=c("all", sort(unique(df$tax)))),
        selectInput("seller",label="choice the seller",choices=c("all", sort(unique(df$seller))))
     ),
     mainPanel(dataTableOutput("table"))
   ))),
  server = function(input, output, session) {
    vars = names(df)[-5]
    vars_selectChoice = lapply(vars, function(name) c("all", sort(unique(df[[name]]))))
    names(vars_selectChoice) = vars
    df_update = reactive({
      out = df
      for (name in vars){
        if (input[[name]] != "all")
          out = out[out[[name]] == input[[name]],]
      }
      return(out)
    })
    updateInput_f = function(penal_name){
      vars_tmp = setdiff(vars, penal_name)
      for (name in vars_tmp){
        vars_selectChoice[[name]] = c("all", sort(unique(df_update()[[name]])))
        updateSelectInput(session, name, choice = vars_selectChoice[[name]], selected = input[[name]])
      }
    }
    brand = observe({
       input$brand
       isolate(updateInput_f("brand"))
    })
    model = observe({
       input$model
       isolate(updateInput_f("model"))
    })
    tax = observe({
       input$tax
       isolate(updateInput_f("tax"))
    })
    seller = observe({
       input$seller
       isolate(updateInput_f("seller"))
    })
    output$table = renderDataTable(df_update())
  }
)
runApp(app)