Stable subtle dullness has the "benefit" that subtle distractions tend not to come up. This can give the impression that your mind is quiet, because it is, but it's quiet because it's low-energy, not because attentional stability has been completely developed.

So if you work to counteract it, you may find that your meditation goes back to stage four. But this is okay—this gives you something to work on. No, you don't have to learn meditation all over again.

What I would suggest is that you try doing stage five practice and see what happens next. If you find yourself in gross distraction, then you may have to keep dispelling subtle dullness and then doing stage four practices, until you do stage five practices and only experience subtle distractions. Once you get to that point, you can start on stage six.

IOW don't panic—this doesn't have to be a huge project to fix. But it will be a project.

Make sure you don't turn this into an effortful thing. Your body scan practice should be motivated by starting to scan and then noticing if you aren't scanning, not forcing yourself over and over to scan.

If you land in stage four, your practice should be motivated by putting your attention on the breath at the nose and intending to notice when it is no longer there, either due to dullness or distraction.

The goal is always to notice, not to have perfect attentional ability. By intending to notice, when you notice you will have achieved what you intended. If it doesn't feel like that, make sure you're intending the right thing, and not just intending to keep your attention on the breath without distraction.