The suitability of breaking up your course would depend really depend upon the content itself and the amount of it.
Students are known to comment when courses are unnaturally split.
If simple terms, if it looks like something should be included which isn't an advanced principle or a next level as such and the course is only stretching throughout a few hours, it's worth considering inclusion. A solid, single, something longer course is often better than several smaller ones.
If you've got a good amount of content and your 'staging' solidified in your mind, I would consider splitting, but disregard the idea of the single certificate since I can't figure how this would be possible. Those potential stages you may have in mind should be enough to warrant individual certificate titles. I think students would possibly be proud of the 4 certificates too (with those areas / stages stated on them) rather than a single certificate.
I've always done what needs to be done for a level, then started on another fresh course for the next adding to both periodically.
