R - dynamically create columns using existing column names in sequence

I have a dataframe, df, with several columns in it. I would like to create a function to create new columns dynamically using existing column names. Part of it is using the last four characters of an existing column name. For example, I would like to create a variable names df$rev_2002 like so:

df$rev_2002 <- df$avg_2002 * df$quantity

The problem is I would like to be able to run the function every time a new column (say, df$avg_2003) is appended to the dataframe.

To this end, I used the following function to extract the last 4 characters of the df$avg_2002 variable:

substRight <- function (x,n) {
  substr(x, nchar(x)-n+1, nchar(x))
}

I tried putting together another function to create the columns:

revved <- function(x, y, z){
  z = x * y
  names(z) <- paste('revenue', substRight(x,4), sep = "_")
  return x
}

But when I try it on actual data I don't get new columns in my df. The desired result is a series of variables in my df such as:

df$rev_2002, df$rev_2003...df$rev_2020 or whatever is the largest value of the last four characters of the x variable (df$avg_2002 in example above).

Any help or advice would be truly appreciated. I'm really in the woods here.