I also chose to do it this way around, as it looks as through there is quite a bit of overlap between BA2/3 and the syllabus in the operational level certificate exams, and so thought I would do them last to keep the knowledge fresh.
I would start with BA1 – there are several elements introduced in BA1 that are revisited for BA2 and BA4, therefore easier to start there in my opinion.
You can however, do it in any order.
I started with BA1, then BA4 now plan to do BA2 and then BA3