v4 is a complete rewrite with a simpler, stream-oriented API. The arc command has yet to be implemented for v4 (please use v3 for now, if you need that).
PR #302 documents the many issues fixed/closed by this new version.
Some of the major new features:
- Stream-oriented APIs (no writing to disk!)
- Simpler APIs (less code and docs!)
- File system abstraction (treat archives like directories!)
- Format identification (by filename or header!)
Many other improvements and simplifications have been made.
The v4 godoc is live. See the README for a tour and code examples. Please try it out and leave your feedback!