The fourth edition of Unix was the first version to have a kernel written in a high level language, C, along with some of the commands. A full and complete copy of Fourth Edition no longer exists. Here is:
The files in nsys/ come from nsys.tar.gz, which was donated by Dennis Ritchie. This is a version of the kernel quite close to that released in Fourth Edition, but without pipes. Dennis Ritchie writes:
This is a tar archive derived from a DECtape labelled "nsys". What is contains is just the kernel source, written in the pre-K&R dialect of C. It is intended only for PDP-11/45, and has setup and memory-handling code that will not work on other models (it's missing things special to the later, smaller models, and the larger physical address space of the still later 11/70.) It appears that it is intended to be loaded into memory at physical address 0, and transferred to at location 0.
The files in man/ come from v4man.tar.gz, which was donated by Dennis Ritchie.
For more information about Fourth Edition Unix, see The Evolution of the Unix Time-sharing System by Dennis Ritchie.