The only predictor we have of how anyone’s MS is going to behave in future is how it’s been in the past - although it’s no guarantee.
Relapse frequency tends to be highest in the early years - i.e. around the time of diagnosis. No huge surprises there - most people are diagnosed when their disease is active, not when it isn’t.
If you have so far been going years between relapses, it is unlikely, but NOT impossible that you would suddenly start having much more. The “average” pattern of the disease is that relapses die down with time, not increase. And yes, in many cases do eventually stop altogether. Unfortunately, that doesn’t mean the person has got better, and their MS gone away. It just means the nature of it has changed, and it’s characterised less by distinct episodes (relapses) and more by continual (but not always fast) deterioration.
But remember, there is no such thing as an “average” person with MS. However, I think you can take some comfort from the fact your relapses have so far been very infrequent. That doesn’t mean things can’t/won’t change, but statistically, it’s a positive indicator.
Relapses don’t run to a timetable like buses (mind you, round here, the buses don’t, either!) So a four-year gap the last time doesn’t mean that’s what you can expect again. And especially doesn’t mean you are “due for one” this year or next, because it’s getting towards four years. It doesn’t work like that!
As others have said, it will happen if it happens. There’s absolutely no way of predicting, and not much point worrying about something you can’t predict, either.
You will deal with it if or when it happens - most people do.