The 4Kw limit should be on the infeed and not the solar panel capacity.
You could put 10Kw worth of panels up just limit the output to 4Kw. Now you have a more stable 4Kw feed.
He bought used panels so the actual input may be a little lower than rated (though it doesn’t seem much lower), but he says he sees some ads for new panels nearly as cheap as he paid for used ones 5 years ago.