Dynamic change in vtree within shiny: How to deselect

This is a follow-up question of this R Shiny: Vtree plot not rendering with Shiny

With this code I can manage to get reactive behaviour for the levels (level1 - level4) and also for the input$values (4,6,8). But how can I get to disable for example level2 (or any other). Or how can I manage to select input$values all or only 4 and 6? With my code so far all levels appear and only one value could be selected!

Here is a reproducible code:

library(shiny)
library(vtree)


# Define UI ----
ui <- pageWithSidebar(
  
  # App title ----
  headerPanel("Cyl vtree"),
  
  # Sidebar panel for inputs ----
  sidebarPanel(
    selectizeInput("level1", label = "Level1", choices = NULL, selected = NULL),
    selectizeInput("level2", label = "Level2", choices = NULL),
    selectizeInput("level3", label = "Level3", choices = NULL),
    selectizeInput("level4", label = "Level4", choices = NULL),
    selectizeInput("values", label= "Values", choices = NULL),
  ),
  
  # Main panel for displaying outputs ----
  mainPanel(
    vtreeOutput("VTREE")
    
  )
)

# Define server logic to plot ----
server <- function(input, output,session) {
  df <- reactiveVal(mtcars)
  vector <- c("cyl","vs", "am","gear")
  
  observe({
    # select only the party columns 1-4; 5 is the county column
    updateSelectizeInput(session, "level1", choices = colnames(df()[vector]), selected = NULL) 
    updateSelectizeInput(session, "level2", selected = NULL, choices = colnames(df()[vector])) 
    updateSelectizeInput(session, "level3", choices = colnames(df()[vector])) 
    updateSelectizeInput(session, "level4", choices = colnames(df()[vector])) 
    # Get counties without duplicates
    updateSelectizeInput(session, "values", choices = unique(df()$cyl))
  })
  
  output[["VTREE"]] <- renderVtree({
    vtree(df(), c(input$level1, input$level2, input$level3, input$level4),
          sameline = TRUE,
          follow=list(cyl=input$values),
          pngknit=FALSE,
          horiz=TRUE,height=450,width=850)
  })

}

shinyApp(ui, server)

As requested.

library(shiny)
library(vtree)


# Define UI ----
ui <- pageWithSidebar(
  
  # App title ----
  headerPanel("Cyl vtree"),
  
  # Sidebar panel for inputs ----
  sidebarPanel(
    selectizeInput("level1", label = "Level1", choices = NULL, selected = NULL),
    selectizeInput("level2", label = "Level2", choices = NULL),
    selectizeInput("level3", label = "Level3", choices = NULL),
    selectizeInput("level4", label = "Level4", choices = NULL),
    # This line is the only change from the original code
    selectizeInput("values", label= "Values", choices = NULL, multiple=TRUE),
  ),
  
  # Main panel for displaying outputs ----
  mainPanel(
    vtreeOutput("VTREE")
    
  )
)

# Define server logic to plot ----
server <- function(input, output,session) {
  df <- reactiveVal(mtcars)
  vector <- c("cyl","vs", "am","gear")
  
  observe({
    # select only the party columns 1-4; 5 is the county column
    updateSelectizeInput(session, "level1", choices = colnames(df()[vector]), selected = NULL) 
    updateSelectizeInput(session, "level2", selected = NULL, choices = colnames(df()[vector])) 
    updateSelectizeInput(session, "level3", choices = colnames(df()[vector])) 
    updateSelectizeInput(session, "level4", choices = colnames(df()[vector])) 
    # Get counties without duplicates
    updateSelectizeInput(session, "values", choices = unique(df()$cyl))
  })
  
  output[["VTREE"]] <- renderVtree({
    vtree(df(), c(input$level1, input$level2, input$level3, input$level4),
          sameline = TRUE,
          follow=list(cyl=input$values),
          pngknit=FALSE,
          horiz=TRUE,height=450,width=850)
  })
  
}

shinyApp(ui, server)
```