Abbreviations like "4u4" and "3u8" refer to different Red Hat versions / releases / updates. They do not refer to multiple CPUs or cores, RAID configurations or memory, etc. However, they do not represent kernel versions. For example 3u8 is not a shorthand for kernal 3.8; that was actually 2.4.21-47 and 4u4 was 2.6.9-42.
Red Hat Enterprise Linux is also referred to as RHEL. The various suffixes of Zoot, AS, Pensicola, ES, Panama, WS and Taroon also indicate different releases.
Wikipedia article on versions:
http://en.wikipedia.org/wiki/Red_Hat_Enterprise_Linux#Version_history
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