Whales, I don't think it is comparable.
Even grading a couple of papers correctly for GCSE can be problematic, as we see every year for remarks, especially in subjects with a subjective element like English. Imagine that a person's GCSE grade is based on every single piece of work they have ever done on anything that they have ever done, right from reception, and that because no one person can look at all of those, lots of different people look at different bits of it, but have a very short time to do so, and sop may rely heavily on the grades given to pieces of work - even though those may not be comparable across different teachers and different schools the child has been to. Then multiply it up a few times. That is, essentially, what an Ofsted inspection is like - trying to encapsulate a school of hundreds or thousands of pupils, in 1 or 2 days, with 1 or a few people, into a single grade.
A different team on a different day would often give a different grade, for different reasons, and the results drift with time (a current 'Good' is graded under harder criteria than an Outstanding from a few years ago, but the old Outsanding grades stand).
Having a 4 level grading process, in which not all schools are included (Outstanding schools, however old their grade, are almost entirely exempt) but where results cannot be compared over time and sometimes between inspectors, implies a level of accuracy that is imaginary, and thus would be better replaced by a 2-level system that does not imply arbitrary distinctions that do not really exist.