Heart rate monitoring sensors and their small LEDs actually don't affect the power consumption at all. Or how else do you think fitness trackers with their constant heart rate monitoring manage to attain a battery runtime of about a week, even though they have significantly smaller batteries? My Watch4 Classic didn't show any difference whatsoever between the three settings. That's an old cliché that was actually never true in the first place. Neither on the sensors from, say, five years ago, nor from today. They were always among the least consuming features, and they become more and more efficient and more precise.
Mine (Watch4 Classic 46mm BT) with AoD enabled between 7am and 10pm lasts two days.
What does affect the battery runtime greatly is LTE, WiFi and GPS. These empty the battery very fast. But the HRM? Doesn't do a thing to the runtime. Even when it also enables the red LEDs at night for the blood oxygenation, which it is also constantly measuring, it doesn't affect the battery runtime to any noticeable amount.
Actually, Samsung has had issues with HRM and other fitness-related features in the beginning of every watch generation for at least the past three years. On some earlier Galaxy Watches (SM-R800 from 3,5 years ago), the sensors either didn't record every ten minutes (and may have killed themselves via software) or had issues measuring continuously in general, even after a ton of updates (there's a thread in the German Samsung Community with about 900(!) posts regarding this, which was the reason why I had to have mine sent in three times, because it always ceased to work after a few weeks), this watch (the Watch4) doesn't properly measure deep sleep, the Gear S3 had issues figuring out what kind of workout you're doing using the workout detection and had a bug that killed the battery in less than twelve hours when an alarm was set, none of those listed properly count the floors climbed... It may as well be a bug relating to the HRM that kills your battery so quickly. Mine doesn't measure properly outside of a manually started workout, but doesn't affect the battery runtime like yours, and issues regarding heart rate monitoring with this watch are actually known bugs since the watch has been released.
Other than that, enjoy your new watch! It sure looks pretty, and since they upgraded the hardware (especially the graphics processor), the interface runs really smooth and is very pleasant to use. As for the other bugs, there's a high likelihood that they are fixable with software updates.
