Galaxy Watch 4 when using the Walking app frequently shows heart rate higher than I am capable of reaching, and higher than actual rate. I sent 3 examples to Samsung Support, all taken during a leisurely walk on level ground. Actual heart rate was 70-75. In one 2 of the examples, it went up to 120. (I have seen as high as 163 -- which can't be done at my age!) In both of the those I paused walking, and in one the heart rate came down to the right level and stayed there. In the other it stayed high. The 3rd example was actually correct, and that is VERY rate. When I stop the walking app and check the direct measurement, the rate quickly comes down to the correct level.
I sent the watch in and the heart rate monitor was replace. I sent it in again (with these 3 examples printed) and got an email today saying it was unrepairable. Now I'm waiting to find out how to get a replacement or refund.