I think this is either a typo or somethings going wrong because last I checked you couldnt get -4 hacks.

Once you hit zero hacks no one can hack you anymore however you can still hack others. So my guess is is that this player attempted to hack others back and failed the hack. If this is the case then their counter would go down again as if they have been hacked.

its all about what was done over 24 hours. if a person rose his action high enough, then got attacked back to the caps, then later on when the original attacks wear off the hostile actions still apply and make it negative.