Re-order dataframe columns in R

Question

I need to re-ordering the columns' position in a dataframe with 500 columns. In fact, I only want the last column to be moved between the third and the fourth columns.

Here is what I tried:

 df[ ,c(1, 2, 3, ncol(df), 4:ncol(df)-1)]

But it gives me a vector of values which are the columns' number. Would you someone tell me what I expect wrong from this code?