As you described the problem, it doesn’t look like you want do reset. The simplest thing for this is:
git add app/models/a.rb app/views/a/index.html.rb config/foo.rb config/bar.rb
git checkout .
so, it adds your 4 files into the index, and checks out clean versions (i.e. discards changes) for other files.
I assume you have not staged (git add) modified files before (you should then unstage it using reset).