Recently I upgraded from Series 4 to 7 and don't regret it at all. I was initially going to wait for S8 because I really wanted that flat design rumored for S7, but due to unforeseen circumstances I decided to give my S4 to a senior family member to use it primarily for fall detection and decided to get a S7 for myself.
I really like the new Midnight color, which is almost black and blends nicely with the display plus I think it looks better than Space Gray. The 20% bigger display is definitely noticeable and a nice upgrade, not to mention the Always-On Display, which was also one of the selling points for me. (I wouldn't upgrade just for that, though.) The SpO2 sensor is nice to have, too, and from my use so far the heart rate sensor also seems more accurate than on S4 during workouts (less spotty).
The battery life with AOD and brightness set to 2/3 and Background App Refresh OFF is about similar to my S4 if not only slightly worse (my battery health on S4 was excellent - still at 95% after 3 years), so if I take it off the charger around 7:30 it will still hold cca. 65% charge around 9pm. With a 45-min workout (and Fall Detection during Workout ON) the battery after cca. 14 hours will be at around 55-60%.
But then if you're content with your S4, I'd say wait for S8. If there is a redesign, it will feel more of an update than S7 to begin with.