A hospital bill was paid in full. Now 4 12 yrs later it resurfaces and now they are saying there is a 1500.00 outstanding bill due because of out of network facility that was used.?

Ask them what they think the statute of limitations is on such bills (2 years I believe). Then note that you were advised the facility was in-network back when all this occurred (I imagine this was the case; else why the settlement back 4 years ago), and note that it's hard to determine if this is negligence on their part, or malfeasance. That could be the underlying message. However, if you find a nicer (yet firm) way to restate this, you'll likely get them to do the right thing and cancel the bill. Note that what matters is the facility's status when treatment was delivered -- not Now. Good luck.