The CAA stipulate that you have to attend an appropriate course of ground school before passing the exams. Bristol GS run such a course and will require you to read the notes, take the progress tests and attend the two brush up courses.
That is the minimum requirement. If you want to do 4 brush up courses and are happy to pay for the extra two then I can't see Bristol turning you away. But you'd do well to take their advice on the best way to tackle it. They have A LOT of experience and know all the pitfalls.
I believe you have a maximum number of sittings, so if you're using four instead of two you're already two down in case you need those extra sittings later.