It’s correct that Metabase is currently using API v3 with GA.
https://github.com/metabase/metabase/blob/master/modules/drivers/googleanalytics/project.clj
I highly doubt that just by upgrading to v4, it will enable comparison - I’m pretty sure it would require a lot of other things done in Metabase.
Currently most other databases have the ability of doing comparison in Metabase through the Custom Expressions:
https://www.metabase.com/blog/Time-Series-Comparisons/index.html
I would recommend that you open a request on upgrading the API, but I doubt it will automatically give the options you’re looking for:
https://github.com/metabase/metabase/issues/new/choose