Git does not match svn or cvs, since it is "completely distributed", so there are actually no clients and server, only nodes. In your case, you want your “client machines” to point to a remote repo on the NAS, where they can push their changes. You can point to this repo using a variety of methods, including a file, as described here:
http://www.kernel.org/pub/software/scm/git/docs/git-push.html#URLS
In git, all your clients are also nodes, so, for example, developer 1 may have an experimental branch in its repo, which developer2 pulls out for review. This branch does not have to be on the "server". It is both confusing and powerful.
The easiest way to start is simply to follow the client / server model. 4 "clients" pushing changes to the git repository on a shared file server.
Is there a precompiled git.exe (like cvs .exe)
Yes, see here, http://oreilly.com/software-engineering/excerpts/version-control-git/installing-git.html
I also suggest registering for a free github.com account, installing git on one machine and playing, deploying other projects, pulling them out, pushing them, etc.
Maybe if you have an hour to kill, buy http://peepcode.com/products/git screencast to get through though git.
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