If you have worked through these topics, you have a pretty good overview of what an AS/400 is.
If you wish to continue your learning experience, I suggest you buy a copy of the book written by Judy Yaeger, "Programming in RPG/400".
The new price on the book is $100, but you can get great deals at www.half.com. Her book is used as the textbook by many colleges. It is a complete study of RPG/400 plus some other AS/400 subjects. Feel free to continue to use your UserID on our AS/400 as you continue to learn.
I love RPG/IV (aka RPG ILE) and will always choose to use it over RPG/400, but if you want to get a job in the AS/400 world, you should first learn RPG/400.
Finally, check out the Basic AS/400 Skills that every programmer should know.