Stronger Crank? - Stock Forged 3.31” vs. Aftermarket Cast New 4”

Frankly, at the power levels you are talking about, a cast 4" stroke crank from Scat will hold up just fine. Will it be as strong as a factory forged piece? Probably not.

But does it need to be?

You need to decide what you want to acheive, and how much you're willing to spend to do it.
Power levels do not automatically increase with a stroker, as the heads hold the key to ultimate hp numbers. But you can step up on the cam size and still have a boatload of torque to hand.