Update version and readme for version 4 release

[aoskotsky-amplify]

Release changes from #319. Bumping with major version change due to the big refactoring that was done but we aren't aware of any breaking changes at the moment.

[tehnrd]

Since this is going with a 4.0 release maybe it makes sense to change the commands to create_domains and delete_domains (making them plural with an s). It would also prepare for #327 . I didn't change these names in order to keep backward compatibility and perhaps it's best to maintain that still.

I have no preference and most use cases will probably be a single domain....so perhaps it's best to leave as is, but just wanted to bring it up as if there was ever a time to change it, it would be at a major release.

[tehnrd]

....or just add both, but internally call the same functions. Ya...that probably makes more sense, that can be part of #327

[aoskotsky-amplify]

I think it's fine to keep create_domain and delete_domain to minimize the amount of breaking changes in this release. In a future release, we could include #323 and deprecate the existing commands.

I guess #327 should be changed so the plugin supports either a map, list or single domain config. It could be either a 4.1.0 or 5.0.0 release depending on how much needs to be refactored again.

[sonarcloud]

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