Thanks folks, I appreciate all the responses.

On rest you have no obligation whatsoever. None. If you didn't get properly notified then you are risking an FAR violation. Even if you and the company do violate the FAR's, odds are no one will know I guess, a long as nothing goes wrong. Then again that depends on how they track/record/audit their systems and all that. Even if you don't fly they could "self disclose" and turn you in as a rogue pilot anyway.

But if something does go wrong and there is a full on investigation, you will be thrown under the bus as careless and wreckless and full on negligent because you knowingly and willingly exceeded your duty day and the company will throw that bus in reverse and come back for another hit, screaming to the high heavens that you should have notified them of the proper start of your duty day and/or called in fatugued, etc. Under the situation you described as you've described it, I would absolutely refuse anything beyond 8pm and let them know as much. Then immediately start calling the union (if you have one) and/or CP office and if there was any resistance, the FAA, and also start looking up red blooded shark wrongful termination lawyers cause I would not be doing that illegal assignment under any amount of bush league scheduler pilot pushing techniques.

Except this isnt a contract question it is an FAR duty time question.

in this situation it would be: "Fly now, lose your licence this evening"

If push comes to shove just remember to never "refuse to fly" you tell them you "can't accept that asignment" There is a huge difference in the hearing. Refusing to fly is insubordination. Unable to accept an assignment is very different.

In your case I'd say your time of notification started at 4 am. You should review the FAQ for FAR117. Although its not law yet airlines are strongly encouraged to implement them earlier and comply with whatever provisions they can ASAP.