Yes there will be a non-Mac App Store version of Dropzone 4 but this is still a few months away at the moment. In terms of upgrade path, I'm looking into providing a discount for existing Dropzone 3 non-Mac App Store customers when this releases. But not promising anything at the moment.
In the meantime, you can checkout the Setapp version - see https://setapp.com/apps/dropzone
This version runs unsandboxed and is more like the non-Mac App Store version. With Setapp you pay $9.99 per month and get Dropzone 4 as well as a bunch of other great apps and it also supports all the developers with their apps on Setapp by giving them some of the monthly revenue.