I personally feel confident that there will not be a big redesign this year. Obviously, I’m just guessing, but I don’t see it happening. They already had to retool things for the series 7. It would probably be very expensive to retool again in just one year.
I think the series 4 was really when it became a mature product. So, from then on, things got more incremental with updates.
All that to say, we’re about 8 months away from the series 8. That’s a long time to wait for things that aren’t likely to change very much. I’d say to get a series 7 now if you want the updated sensors (as someone else said, the current generation optical heart rate monitor is definitely more accurate), always-on display, SpO2, bigger display, faster charging. Those add up to a pretty nice upgrade from a series 4 in my opinion. Only you can know if it’s worth the cost to you.