To define a new method for an S4 class of object, first you need an object of class S4. Here is the code to create an object of S4 class robustSummary2:

    x <- c(13, 21, 19, 18, 21, 16, 21, 24, 17, 18, 12, 18, 29, 17, 18,   
    11, 13, 20, 25, 18, 15, 19, 21, 21, 7, 12, 23, 31, 16, 19, 23, 
    15, 25, 19, 15, 25, 25, 16, 29, 15, 26, 29, 23, 24, 20, 19,
    14, 27, 22, 26)
    robSum <- function(obj){
      med <- median(obj)
      mad <- mad(obj)
      q1 <- as.numeric(quantile(obj, probs = 0.25))
      q3 <- as.numeric(quantile(obj, probs = 0.75))
      return(list(median=med, mad=mad, q1= q1, q3=q3))
    }

    rStats <- robSum(obj=x)
    rStatsS4 <- new("robustSummary", median=rStats$median,  
    mad=rStats$mad, q1=rStats$q1, q3=rStats$q3)

    > isS4(rStatsS4)
    [1] TRUE

Now, you have an object rStatsS4 of class S4. The task is to create a new method for this class to display the output in a customized structure.