How to pick the most frequent values (mode) from a raster stack

I have four layers in a raster stack and want to pick the most frequent value in each cell among the four layers.

Here is the dataset and code:

require(raster)
a <- raster(matrix(c(12,11,11, 
                     NA,11,11,
                     11,11,13),nrow=3))

b <- raster(matrix(c(12,12,12,
                     NA,12,12,
                     14,12,13),nrow=3))

c <- raster(matrix(c(13,9,13,
                     NA,13,13,
                     13,NA,13),nrow=3))

d <- raster(matrix(c(10,10,10,
                     NA,10,10,
                     10,10,10),nrow=3))

stk <- stack(a,b,c,d)

I wonder if it is possible to do it with code something like?

which.freqV <- function(x, ...){
   ???
}

max <- stackApply(stk,1,which.freqV,na.rm=NULL)

I had to this in my scripts (calculating the mode of a stack) and found something that works perfectly:

  Mode <- function(x) {
    ux <- unique(x)
    ux=ux[!is.na(ux)]
    ux[which.max(tabulate(match(x, ux)))]
  }
  layers_mode=calc(stack, fun=Mode)

Now, if you want to calculate the frequency of the mode for each cell, use this:

  ###calculate proportion of layers that have mode value as measure of uncertainty
  Mode_count <- function(x) {
    ux <- unique(x)    
    ux=ux[!is.na(ux)]
    mode=ux[which.max(tabulate(match(x, ux)))]
    sum(x==mode, na.rm=T)
  }
  layers_mode_freq=calc(stack, fun=Mode_count)