Yay, new watch!
However the Watch 4 is not without some limitations, and it isn't the perfect fitness or medical device, but I think fits into the "good enough" category for all of these things for me. I chose the Galaxy Watch 4, 44mm, not the "Classic" model which was heavier and more expensive.
No Samsung Phone? No features!
Samsung has artificially limited this watch to have features only available on Samsung devices, however, there are relatively easy-to-install apps for both the phone and the watch to allow any Android phone to use the watch to its full potential. No modifications are permanent, none will interfere with watch updates nor official Samsung software updates, etc. These seem to be very safe to apply and use. The things the modified files fix (and some they don't fix) are documented in this handy Reddit thread: What is the downside if you use a Galaxy watch paired with a non-Samsung phone?
Read the thread above and if you choose to install the modified files, the files and instructions are here in the Mod developers' Google Drive.
I did opt to install the modified files and once I applied and configured them I am able to use all watch features in the US on my Pixel 6. EKG and Blood Pressure are the two primary ones that are otherwise blocked as noted in the Reddit thread linked above and these both function on the watch when they otherwise would not.
Overall fit and function
I wasn't sure about the watch band having the band tucked under itself but in practice it's really nice not having to move a loop around to capture the flap, having it catch on things, etc. But you'll likely want to wear the watch one notch looser than you think you'd want it because the strap takes up some space.
Overall the watch is very responsive, the watch itself is very low profile, not flashy, light, and comfortable. WearOS has improved a lot over the years, still not really enough but there is constant improvement.
When I first got the watch it had 2 OTA updates, both I seemed to have to initiate manually. One took about 20 minutes, the other about 10 so they didn't take a lot of time, but every time I tried to start the update from the watch it would fail. Starting them from the phone seemed to work fine. This update process is really clunky and honestly hasn't gotten any better over the years. Google really needs to work on this process. Maybe if the watch/phone discovers an update and does it in the background the user will never know or care, but if you want to force the watch to get updates it's a pretty disjointed process.
After the updates were done several apps had to update from the Play store and 10 or so from the Samsung store. This was slow but fairly painless.
I will likely be doing some nerdy measurements here soon but overall I'm happy with the battery life. It would easily last all day with the heart rate monitor set to 1s updates, stress measurements, Always on Display, etc. The watch charges at about 1% a minute which isn't particularly fast but it's plenty fast enough for me. I will generally wake up, put the watch on the charger, eat breakfast/shower/etc and the watch is 100% when I put it back on. I wear it the rest of the day, evening, and sleep in it.
Use as a medical device
Having a watch that can do heart rate monitoring, pulse-ox, blood pressure, EKG, etc, sounds great but keep in mind that this is not medical-grade equipment. A watch is at best a good second medical device, but do not use this as your only medical device. Blood Pressure measurement is inaccurate and needs frequent re-calibration and the EKG is of limited use and only does manual tests when you initiate it; there is no constant monitoring of your heart health. That said, the pulse-ox and heart rate monitor are pretty good. The body composition evaluation is neat but no idea how accurate it is and just what I need; my watch fat-shaming me.
No matter what, this watch does not replace yearly exams and if you're at high risk, devices like a Blood Pressure Monitor, pulse-ox, a real EKG strap, etc, can be a requirement to monitor your health. Take medical advice from your doctor, not from your watch.
To expand on the Blood Pressure monitoring, I calibrated the watch when I first got it and kept getting fairly consistent results when compared to my Omron blood pressure monitor, however, at some point I forgot to pick up my blood pressure medicine for a few days and when I measured my blood pressure it was higher than it normally is at 140/90 on my Omron monitor (take your medicine, kids!) but unfortunately, the watch continued to report 120/80.
I did a re-calibration of the Blood Pressure on the watch and after that, it reported around 140/90. A few days later after I had regularly taken my medicine and my Blood Pressure was back to normal at around 120/80 the watch was still unfortunately reporting 140/90.
Another data point; a few days later I was feeling a little lightheaded and my Omron measured my Blood Pressure at 105/70 and the watch was still reading around 120/80.
This isn't a lot of data to go on but my testing makes me not trust the Blood Pressure readings at all. The watch does not seem to be doing anything beyond showing values around whatever you calibrated it to. No wonder the FDA won't approve its use.
Use for sports
For working out the Heart Rate monitor is okay in some areas, not great in others, casually it's likely good enough for most folks. Optical heart rate monitors are good until the watch is moving, vibrating, you're sweating, etc. It may be misread as half the real rate, double the real rate, or just completely not read at all. If you're training or highly competitive you likely will want a Polar H10 or some other more accurate and consistent device. For a highly detailed view of how accurate the Heart Rate and other functions are, I recommend watching The Quantified Scientist on Youtube, he puts out videos on many popular devices and has several videos on the Samsung Watch 4.
Not all is perfect
Tying features to only Samsung devices feels like Samsung trying to drive more Samsung phone sales which is just silly. Open the watch features up to all phones.
The buttons should be more configurable. You can configure them, but you really can only assign one custom app to a button. The other buttons/configurations let you toggle between Bixby or Google Assistant, etc. I would prefer more options, with two buttons you have to press, hold, and double press, potentially 6 total unique actions you can do, but in reality, it's relatively limited.
Samsung Pay only works if your phone is 100% unmodified stock with a locked bootloader. My Pixel 6 has an unlocked bootloader, so Samsung Pay refuses to work therefore I can't make payments using the watch either. Not a feature I think I'd use often but it could come in handy in a pinch.
This is my fourth WearOS watch, my first one lasted a few years, second two lasted about three years (I got them sort of around the same time) but all of those have stopped charging or functioning and I've been without a WearOS device for a while. I've been wearing an Amazfit Band 5 for about a year and it works pretty well but it has a very limited function beyond a fitness band. I'm glad to have a WearOS watch again and hope to see Samsung continue to put out updates to this watch and continue making new ones.
I like to keep my electronics looking as good as they can for as long as I can, so I use a case on my phone and I almost always buy a tempered glass protector for my watches. However, the design of the Galaxy Watch 4 seems perfect for a tempered protector as it's very flat, there was an alternative I wanted to try out, and that was a bezel you put on the watch which protects the edges and gives it more of a "real" watch feel. Ultimately I feel the bezel doesn't fit with the watch quite right and it interferes a bit with swiping and overall wish I hadn't tried it. But the biggest problem with the stick-on bezel is the STICK part. Every morning my watch is covered in fuzz that has stuck under the bezel where the sticky tape is holding the bezel down. I keep digging the fuzz out and cleaning it and hoping that eventually the edges of the sticky tape lose their stickiness and the fuzzy watch face effect stops happening. Gross. After bout a week this problem has mostly gone away, but a few random lingering hairs and things I still keep having to pick out. I think over time the problem will go away, but still, yuck.