No, I would not take a 1st for Dak and a 4th round pick. That would be a terrible trade for Dak.
I would want what the Rams gave up for Matthew Stafford. The Lions got Goff, 2 first round picks and a 3rd round pick for Stafford. Why would the Cowboys take less than that? Of course teams know Dallas will lose Dak of they don't re-sign him, but they would still be competing with the rest of the league if he becomes a free agent. I am pretty sure if the Cowboys put Dak on the market they could get 2 first round picks and more.
But again, Jerry let this go on too long. He should have done something with Dak before the draft. Same thing with Lamb. If Jerry was unwilling to pay Lamb $34 million in February, why would he be willing to pay him that much now? Or the other way around? The time to trade both of these players was back in March or early April.