Yep. We can debate how much.
That's the question, isn't it. Not really. He needs to shoot well enough that they need to guard him. The problem is that the stretch 4 is usually going to be on the wing/corner area of the court (the right side typically) and Jackson probably won't, because he's a facilitator on offense and we want him up to. So no, he won't be a stretch 4 on offense, but he'd guard the 4 on defense.
This is one of the reasons I think he will play less at the 4 than some people. I think we will find out it isn't really working, because Alleyne would actually need to play the stretch 4 on offense. Johnson and Karaban can be stretch 4s. They can both shoot and play that corner/wing position on offense.