An ssl certificate provider used 4 as their private key. This should be hundreds of characters long. Somebody figured this out and was able to issue their own certificates based on it. This results in loss of trust in their certificates which is a big deal.

It is compared as bigger than one of the largest security issues in IT the last couple of years since it forces everybody to patch their shit to not trust this certificate chain. And lots of stuff aren't patched that easily.