To get details on the S4 class, go to https://cran.r-project.org/doc/contrib/Genolini-S4tutorialV0-5en.pdf.
You can access elements of S3 objects using the $ sign. This is very similar to accessing columns from a data frame. However, to get access to individual items from an S4 class object, you must use the @ sign, for example, object@slot1.