4 hours is a perfectly reasonable response time for an email. It’s not IM

But not for responding to a question on a call!

That's one reason calls can be superior in some situations.

Right. The point is that reasonable response time can turn a 10 minute conversation into a 48 hour long conversation that requires me to context switch 11 times over two days instead of just once.

If it's a straightforward product that might not happen. If it's a product with lots of subtle complications and I need to ask lots of questions whose answers depend on their answers to previous questions it will definitely happen.