In post number two above, I included a link to the Thorn step through frames. This trip log was just posted on Crazy Guy, one of the two bikes was that step through frame made by Thorn.
https://www.crazyguyonabike.com/doc/...id=491382&v=4n
Although they only used rear panniers on the step through bike, I suspect that it would have handled four panniers just fine.