Defining binary indexing operators

Here is an example. Note that $ does not evaluate y so you have
to do it yourself:

x <- structure(3, class = "myclass")
y <- 5
foo <- function(x,y) x+y
"$.myclass" <- function(x, i) { i <- eval.parent(parse(text=i)); foo(x,

i)

}
x$y # structure(8, class = "myclass")

If I got it right, in the above example you provided '$' is defined as a 
method for a S3 class. How is it possible to do the same with a S4 class. If 
this is not possible, what is the best way to define the '$' operator whose 
first arguments is a S4 object and doesn't overwrite the global definition?