Mainframes are large scale Z series with job control language, extreme fast speed.
AS/400 is mid-range i series system with data control language, slow speed for cost budget offices. They are very secure and mostly used in banking, insurance and financial institutions to run their applications.
AS/400 is now know as system i. It was also known as i series.
1988 - AS/400
2000 - e server i series
2004 - i5 Servers
2006 - System i
2006 - System i
2008 - IBM power systems