To be honest I don't think you can migrate them.
What you could try is create 1 writable on 4.0, it will then create a new folder on the datastore which is called Appvolumes\writable.
You could then try and move (or copy, that might be a better idea) from Cloudvolumes\writables to that folder and import the writables in 4.0.
To be honest I have no idea if this is actually supported.. But you woul have to ask GSS for that info.
I am going through the exact same process right now as well. The fling does work for AppStacks, which we have confirmed here. But, we need a way to migrate Writable Volumes as well. I combed through the documentation and I see no mention of this anywhere!
I have tried copying over a Writable from 2.x to 4.x and placed in to the new "writables" directory on the datastore and clicked Import in the new AV 4.0 Manager, however it does not see it and does not show up in the Manager.
Looks like we may need to get ahold of support on this one, can't see how this is done. Hopefully there's a way so that we don't have to manage existing Writables in two separate places (in the new Inventory tab AND Volumes 2.x tab).
This is a big problem for me if I can't migrate my 2.18 wrtiables to 4. I'm going to put a feature request for this.