System V, Release 4
An AT&T release of UNIX (the trademark) from which most "System V"-like features and conventions are descended.
The two major schools of UNIX (the theory) lineage are SVR4 and BSD. While both contribute toward POSIX compliance, most UNIX-like operating systems lean towards one or the other.
From the system administrator point of view, a good example of the difference between the two lies in the boot scripts. SVR4-based systems have a complex series of scripts which do not necessarily associate 1:1 with run levels the way boot scripts do under BSD-type systems.
From the user point of view, a good example of the difference is in the ps(1) command. To get similar data, you run ps -efl on SVR4 systems and ps aux on BSD systems.
From the programmer point of view, a major point of dissention between the two involves the peculiarities of how the various signals are generated and handled.
SVR4-like Operating Systems: