Compress 4 months archive files

You can use ZIP with the .NET library #ziplib:
http://www.icsharpcode.net/opensource/sharpziplib/[^].

Another option is using SevenZipSharp, a .NET wrapper of the famous 7-Zip. This library will allow you to choose from a good set of compression formats. Please see:
http://en.wikipedia.org/wiki/7-Zip[^],
http://sevenzipsharp.codeplex.com/[^].

Both ZIP libraries are open-source.

However, I am not sure you really need compression for archiving. If this is your code archives, it would be much better to use one of Revision Control Systems:
http://en.wikipedia.org/wiki/Revision_Control_System[^].

Practically, if you don't use one, consider all your asserts highly endangered. Even if you archive, you can get lost in those archive. It's very typical that developers of a good team commit code to the code base after each small consistent modification, sometimes, many times a day. If you are not doing it for your development, even if you develop along, you should stop doing what you are doing and establish the use of such system. Please see this discussion:
Revision control systems, which to choose from?[^].

—SA