In Red Hat Enterprise Linux 4, ES and AS (Enterprise Server and Advanced Server) were supported for different platforms and server hardware.
AFAIK they have the same code base and have almost 99,999% the same RPMs but differ in the way Red Hat supported them.
Now there is only the RHEL Advanced Platform (which is used as cheaper subscription model for clusters, virtualization and bigger servers) and the standard RHEL.
Currently there is RHEL 4 U7 and RHEL 5 U3 on the market. But usually you will get only RHEL 5 to buy.