We could use an alternate gemfile or install it and put it on the load path ourselves.  That said, Minitest 4 is legacy at this point.  We don't need to explicitly not support it through an error or anything like that, but my position is that we shouldn't put any effort into minitest 4 support and tell people it's not officially supported (at least, not with `expect_with :minitest`....users can still do `expect_with MiniTest::Assertions` themselves).  If we supported it we'd have to support it until RSpec 4 and I don't want to put that burden on us.