Iron Filings
There is no particular evidence that these were metallic iron, merely something dark and magnetic - probably a magnetic iron oxide.
Also, they weren't intact fragments 2-4cm long, but strings of smaller particles held together by magnetism - just like when you pick up iron filings on Earth and they form "whiskers" far longer than the individual filings.
Of course, if you can offer a *good* model for why long fragments of iron are common on Mars, we'd love to hear it...